I'd only expand when I see my opponent expand. At platinum level, usually whoever expands first loses, but especially if they are protoss.
If Terran is giving you grief you need to think about two things.
1. Are they going bio or mech.
2. When are they going to push out.
Now remember if he's going mmm or massing 4-5 tanks and some bio, he will be mining out his main around about the time he pushes out. This means if you can stop his push, and have enough units left over to deny him an expansion, you have won the game. Remember this key point, it's probably the most important one.
Chargelots + sentries own marauder heavay mixes. if you scout medivacs as well, add a templar archives as soon as you can but ignore storm for now, just focus on getting 2-3 templars out. Feedback will make his 3 or so medivacs a waste of money (i now like seeing medivacs from terran, it means 1 less maurauder
).Wait for him to come up your ramp, cast force field when half his troops are up and then engage. Even if he somehow gets everything up the ramp, an equal number of chargelots with a handful (2-3) sentries will win the engagement decisively.
Once you've held the push, warp in 4 stalkers, and along with your remaining templars/sentries/zealots and a probe, counter push building a pylon out the front of his base... He might have a few defensive forces but you should be able to handle it. If, somehow, he has two+ sieged tanks there and a bunker. Then just pull back, get an expansion up, add a stargate and another 2 gateways and just void rays, research storm and amulet for your templars and chargelots. With a few stalkers you can contain him until he has to force the situation himself by busting out to take an expansion.
Vs mech, you can normally kill this right at the start of the game with 3 fast stalkers. Most platinum and low diamond Terrans wall in with supply depots and you can pick these off with your stalkers forcing him to engage you with whatever marines he has. 3 stalkers can handle a marauder too. If he has 2 maurauders, then break off the attack and head back home. Make sure you control the xel naga towers though.
Against meching terrans I tend to favour 2 gate robo into twilight 2 more gates. Usually the Terran will do a cute 2-3 rauder marine push REAL early you'll just have time to get a couple stalkers and a sentry out. use the sentry to ff your ramp until your first immo pops out, then clean the rauders up.
Now research blink, add a couple more gateways, and as many immortals as you can afford (to around a maximum of 5). When you have a nice big force push out occupy the middle of the map. Occupy watch towers, have a probe out near his ramp, use an observer to keep tabs on expos. Basically use your map control to deny him an easy expansion, and when he pushes out, attack him on the move. The observer should give you the positioning info to do this. If he sieges up, blink a single stalker at a time into the middle of his bio support so his tank fire kills off his own units. Then when the bio support is low, blink surround the tanks and move your immortals into clean up.
Once you defeat the push, take an expo, add a couple extra gates, and tighten the contain. You might even be able to blink past his wwall with your remaining stalkers and end it right there
