He probably understands you, but you can check it out. Ask him something like, “Do you want a cookie?” Then, if he nods yes, offer him a glass of milk and notice what he does. If he accepts the milk (or whatever alternative you offer) without protest or a look of puzzlement, then he probably didn’t fully understand what you said to him. He may have understood only the “want” part of the question and has learned that, after he hears that word, something good will be forthcoming. If he frowns or shows some other indication that you haven’t fulfilled your bargain, then he probably did understand your question.
“Where” questions are good for checking understanding. When he is sitting in his high chair or you are holding him in your arms, ask “Where’s the light?” As he probably can’t point yet, note whether he looks at a lamp or the ceiling light. Do this with other words he knows—the refrigerator, the dog, the car (does he look toward the door?). And be sure to include a few things that aren’t there, like “Where’s Grandma?” If she isn’t present, do you get a puzzled look (or a glance toward the door)? Also, put a few toys out on the chair tray and ask him to do certain things with them: “Put the block in the cup.” (For that one you should ideally have on the tray both a cup and a box.) “What do we do with a cup?” (Does he pick it up and appear to drink out of it?) You can make these up by the hour, and both of you will have fun. And before long, he’ll be giving you verbal answers.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.