Here are a few examples of infantilization. They are by no means the only examples, but they can provide a framework to get you thinking:

One partner has to routinely try and calm the other partner down, because they have mood swings, or start panicking.

One partner is not the intellectual equal of the other partner, and dumbs things down for them.

One partner tries to avoid saying things which might upset the other's ego, because it might start a fight.

One partner is fine with the other seldom thinking for themself and deferring to them for opinions and decisions.

One partner has to constantly remind the other of things they are neglecting to take care of.