You may have noticed, by default, it's a pain in the ass to have to hold down the left analog stick to crouch, not to mention accidently tapping the right analog stick during a heavy firefight to have your weapon zoom in and you be unable to fire it. These problems can both be fixed by modifying the controls.
Firstly, pause the game, go to Options, then Controls. Select Toggle as ON for crouch, and Toggle as OFF for zoom. You'll notice the effects immediately. Now, go back to the menu, hit 'RESUME GAME' to return to where you were in-game, and then save your game to retain these settings for next time you play.
Grenades have a lot more uses that you initially may think. To throw a grenade, equip one from your inventory, and pull down the right trigger. This will perform an over-arm throw. The grenade will explode after 5 seconds, and it can even be thrown at wall to bounce around corners.
Alternatively, you can crouch and press the left trigger to roll a grenade along the ground. This is only needed in a few situations, but it's nice to know it's there when you need it.
The final method of using a grenade is by running with one in your hand when something is chasing you, then tap the left trigger to drop it. Whatever is chasing you (eg - zombie) will follow after you, then walk over the grenade you dropped and be blown up.
The combine will occasionally roll/throw grenades at you, so quickly equip your gravity gun, pick it up and throw it back at them, to watch their own grenade explode in their face.
The vehicle you use throughout Highway 17 is far more usefull than you give it credit. Here are some tips on getting the most out of it.
1. Use it for cover. When a gunship is bearing down on you, or circling back and forwards, and you are in the middle of an open road, park it sideways across the road and crouch down right next to it. The car will absorb all bullets which the gunships can fire, and while it reloads, jump out and take your best shot at it.
2. Use it as a weapon. When you are surrounded by a huge amount of enimies, whether they be antlions or combine soldiers. Run them over, while using the built-in gun to eliminate others which are approaching. When using the crane, you can also lift it up, and drop it on enimies. Be creative and you'll be amazed in how many ways you can use it.
3. Use it's supplies. There's a crate of grenades attached to the back of it, so when you're running low, make sure you use it to top up.
Here are some explainations/tips about the weapons of Half-Life 2, their uses and a few tidbits of info to help clean out the Combine.
Crowbar- Freeman's favorite headbuster will serve you well during the entire game, whether it's bashing supply crates or headcrabs. There's also no better weapon when crawling through a vent duct. Always keep it handy.
9mm Handgun- every FPS has one, but the pistol in HL2 is a little more than what it seems. You'll use it alot during the first portion of the game as ammo is plentiful and almost everywhere. With a fast rate of fire, what the handgun lacks in stopping power, it more than makes up with its accuracy. Use it mainly on smaller enemies like headcrabs, and keep an eye out for Combine troops who are too far away to hit you as the handgun will tag them from almost any distance.
SMG- the SMG is a close-range bullet hose, capable of a blistering rate of fire. Like the Handgun, ammo for the SMG is abundant, so use it as much as possible to conserve ammo for heavier weapons. The SMG also has a grenade launcher attached, which is absolutely devasting to Combine squads and bigger monsters with a little practice. The grenades, however, don't show up much, so it's best to conserve them unless otherwise nessesary.
.357 Magnum- powerful and loud, the Magnum is a great fall-back weapon. A few shots with it will take down almost anything, while a headshot to any Combine soldier spells a one-shot kill. Another great feature in addition to being powerful is its accuracy, as you can blow a Combine off a roof with a single round! For those looking for a good time, a close-range headshot to a Combine will blow them in a backflip.
Shotgun- the shotgun will be your main workhorse throughout most of the game. Any idiot could tell that its deadly in close-quarters, but its uses don't stop there. Unlike shotguns in other FPSs, Half-Life 2's shotgun is useful at engaging targets at almost any range. The secondary lets both barrels go at once, making it a ribbon-winning death-dealer. The only drawback is its small magazine and slow reload time.
Pulse Rifle- long-range precision, stopping power and a quick reload are the Pulse Rifle's claims to fame. While full-automatic fire is great, learn to fire in bursts of 2-5 shots to reap the full benefits of its amazing accuracy while conserving ammo. Though, the best thing about the Pulse Rifle has to be the Charged Pulse. This secondary rivals any other weapon, including the Rocket Launcher and Gravity Gun. Using them in confined areas is almost like cheating, as the Charge bounces around and ionizes anything in its path. Even in the age of the PS3 and 360, it's still one of the coolest things you can witness!!
Sniper Crossbow- your only sniping weapon, the Crossbow takes a bit of practice to master, but once you get the hang of it, you'll wonder why Battlefield and MW3 don't have one. A weapon that's quick on the draw, the Crossbow should be used accordingly, as ammo is limited and the Combine are notorious for dodging bolts by mere inches. Be quick with your shots, and you'll be sticking troopers on the walls like you're hanging pictures.
Grenades- the most versitle weapon in your arsenal other than the Gravity Gun, the physics of the game make the grenades useful and funny at the same time. Use them to breach rooms before going in, clearing debris and scattering Combine squades. Grenades are also helpful against some of the tougher monsters that don't move as fast, and also taking down stationary turrets in Nova Prospekt.
Rocket Launcher- a no-brainer, the Rocket Launcher is a devastating weapon when used properly. Using a laser-guided system, the Rocket Launcher is the only weapon that can take down a gunship. While firing straight at a gunship almost certainly gets your rocket shot down, the real strength of the weapon lies on your ability to guide the rocket towards the back of it, where the gunship is most exposed. Wait until it does a fly-by, then fire freely, using the laser to bring the rocket in for a hit. 5-7 shots will bring any gunship down.
Antlion Pheropods- throughout the chapters Highway 17 and Sandtraps
you'll be constantly hounded by the Starship Trooper-esque Antlions. However, upon meeting and defeating one of the Hive Queens, you'll be rewarded with these unique weapons. Now you have total control over an unlimited number of Antlions using their bug bait. While they serve no real lethal purpose, a well-placed pod can stun Combine troops, giving your Antlions a few seconds to close in. They become rather useless after Nova Prospekt, they can still serve you well. Combine troops are good at dodging grenades, but stun them with a Pheropod, roll in a grenade, sit back and watch the bodies fly.
Gravity Gun- as infamous as the BFG-9000, the Gravity Gun simply has too many uses to list, so I'll just put up a couple of the more defensive properties of it. When under heavy fire, one of the best thing to do is to grab an oil drum, crouch and move slowly towards your targets; you'll still take a few hits but they're minor. Another great feature is the ability to erect a barracade out of assorted junk any time you think you're about to be ambushed.
There you have have it. Now get in there and start peeling brass like bananas!!
To get full ammunition for your selected weapon press Y B A X White Y X A B White
In Entanglement, where you are fighting with Alyx, let her run through any load screen first, and then wait 15-25 seconds before following. When you follow, a duplicate of Alyx with pop up right in front of you. After the loading, there will be the pop up Alyx right in front of you, and the real Alyx a bit past her.
Not only does this look weird, but you now have twice the firepower to cover your back, although it won't last long. Let them both take the initial fire of the combine in any upcoming battles, but keep and eye out for which is the real and which is the copy, as it doesn't matter if the copy dies, but you must keep the real one alive.
NB: This is a nice little glitch you may or may not come across, it boils down to luck really. However, this is a rare glitch, and not a well documented tip for getting extra firepower, so be thankfull if you're lucky enough to get it.
During gameplay enter: Up(2), Down(2), left, right, left, right, B, A
Left x4, Black, Right x4, White- Unlocks all chapters and makes them playable