Learn Italian: Non sto bene, vado a letto. - I am unwell, I am going to bed.

In English we use the verb 'to be' for how we feel, but Italian uses the verb stare (to stay) when talking about health and well-being. When describing your health with an adverb like bene (well) or male (badly), you must use stare. So sto bene means 'I am well', and non sto bene means 'I am unwell'.

What is the dictionary form of sto?

It comes from the verb stare. It is an irregular verb, which is why the 'I' form is sto instead of following a normal pattern. You will use this verb a lot for greetings and health, like in the common question Come stai? (How are you?).

Where is the word for 'I' in this sentence?

Italian usually drops the subject pronoun io (I) because the verb endings already tell us exactly who is doing the action. Both sto and vado are specifically the 'I' forms of their verbs. You could say Io non sto bene, but leaving io out is much more natural.

Why does it say a letto instead of al letto (to the bed)?

Just like in English where we say 'going to bed' instead of 'going to the bed', Italian drops the word for 'the' in certain common daily phrases involving places. The idiom for going to bed is simply andare a letto.