I presume you are asking about why the infinitive sometimes has 'to' before it, and sometimes doesn't, as in why we say 'I want to go' but 'I must go'?
In fact, it's easier to tell you when you DON'T put 'to' before the verb. Two common situations where you don't put 'to' before the main verb are when it follows a modal verb or a verb of perception.
Modal verbs are: can,could, will,would, shall,should, may, might, and must. For example:
I can't go
Will you go?
He should go
May I go?
Verbs of perception are: see, hear, watch, listen. For example:
I watched him work.
We heard them sing.
In most other cases, the infinitive needs 'to' before it.
He wants to go.
They decided to go.
She told me to go.
I hope that helps.