Donald Trump's poor grammar is typical of what a young child would use. A recent study put his speaking at a level of an 11 year old. He frequently speaks improperly. and has a limited vocabulary. This is probably done intentionally in order to appeal to his base of supporters.
Trump has a way of saying a lot of impressive words without saying anything. He just speaks in generalizations about how we're going to be great again, but he doesn't offer any kind of in depth plan for how that's going to happen, exactly. He doesn't even use complete sentences. He doesn't think through his thoughts, he just says whatever comes to mind.
Donald Trump's use of grammar is directed towards the masses, who aren't all Harvard and Yale graduates. The way he speaks appeals to the common masses who are fed up with the establishment politics and elitism that goes along with it. If there is a problem with the way he talks, the voter is the reason why.
Donald Trump is incredibly bright so don't be swayed otherwise. He uses words to get people riled up. It may seem like elementary verbiage but he knows exactly what he is doing. He is playing to a specific demographic. He wants to get people angry and passionate about what he is saying and doing. He's like the pied piper playing the right tune as his followers follow badly.