The meaning of adding "to" in front of a verb does not change the MEANING but it changes its USAGE. Example: 1) I am going TO FILL the cup 2) I am FILLING the cup You never use the verb "fill" or "feed" without to or "ing" with the word or before it. Message me if you are still confused.

Adding 'to' is making the word an 'Infinitive', which means that you cannot say someone is doing it, but can be used with the future tense. For example, you couldn't say 'He to feed his cat', you would say 'He fed his cat'. However, you can say 'He went to feed his cat' or 'He is going to feed his cat', meaning he did/went to do it in the past and future tenses respectively.