One very effective strategy to gain the advantage is too attack him with certain units that force a building or training or action from him.
Example: If you simply get some Dark Templar and very obviously send them to a spot and start harassing, that forces your opponent to start building detection, thus wasting some resources and time in getting it. If you time it right, you can even attack him while he is in the midst of this transition. Attacking while your opponent is teching to a new unit, researching a vital upgrade, or building defenses is very important. You can catch your opponent off guard while he is waiting for that upgrade to complete before he pushes.
Another way I found was through a match a few weeks ago on Scrap Station. The ground distance to our bases was far, however I like to take advantage of the short air distance by sending in warp prisms to deploy and warp in troops. He saw this and began to build Missile Turrets, EVERYWHERE. I’m not talking about 2-3 here and there, I’m talking about at least 5-10 for each of his expansions (he had 3 by the end of the match). I didn’t have a backup strategy back then so I continued to try and do the same thing with little success. What I should have done is take advantage of the fact that he was pumping in at least 2k+ amount of resources for this defense and started just mass building units to try and break this Terran who was by now turtling. Its important to remember this key fact: If he starts mass building these defenses, he is not building units. With all those Missile Turrets, an army consisting of mainly Gateway units and Immortals could break the line and would have probably won me the game.
The game ended with him building up several Battlecruisers and sending them at me after I attempted to use a Mothership and Mass Recall inside his base.
The key to begin to start controlling your opponent is scouting. If you don’t know what he is doing through scouting or early aggression, it is very difficult to gain the advantage. I am interested to learn other ways of controlling your opponent. I’ve only played a total of 40 games so I still have a lot more to experience.