EPT
There's an issue with early-game PvT where you need to decide for certain if you're going to build cannons or not based on your scouting information. When do you need to build cannons and how should you position your units?
Refer to one of my older blogs about scout timings: http://www.teamliquid.net/blogs/viewblog.php?id=423048. In this blog, I came to the conclusion that a MsC scout or a zealot/stalker/MsC poke @5:30 was all the necessary scouting needed in PvT. So what are we looking for when we enter the terran's base?
I like to divide what the terran can do into 2 basic categories: early starport vs. later starport. In all starport scenarios, we know that a drop can be incoming in the back of our main and that very often we will also need detection in our mineral lines. Therefore, if you scout a starport, build a cannon in your main mineral line. If you don't, you know you're safe to start upgrades and adding on extra gateways. If your opponent is conspicuously missing some things, it's safe to assume things like proxied factories or starports or perhaps a hidden 3rd CC which you can attempt to scout with a probe + initial stalker.
The reaction to drops is very simple: place a cannon in your main, place your stalker/sentries in your main between your nexus and the likely drop angle, and rally an observer to your MsC. If your opponent attempts a frontal assault with marines/widow mines, you can simply use the nexus cannon to ward it off. Meanwhile, in your main, your mineral line will be safe from pressure as long as you don't lose your expensive gateway units. This defense also works well against banshees as well as the old 8 marine drop/4 hellion runby strategy.
(NOTE: Far too often, I've seen pros mess up this defense by trying to pick off the medivac at the front or trying to swap the positioning of the MsC vs. units, and they all generally get owned by marine/widow mines, especially if the mines can burrow on the ramp and split up units. I'm not sure why so many pros defend drop-based aggression so badly.)
