Know how + infinitive

Know cannot be followed directly by an infinitive. We use the structure know how to.

I know how to swim. (NOT I know to swim.)

In a formal style, know can be followed by object + infinitive.

I knew him to be a sincere person.

But note that this structure is rather unusual. Instead, we use that-clauses.

I knew that he was a sincere person.

Tenses

Know is one of the few verbs that cannot be used in the progressive form.

I know what you mean. (NOT I am knowing what you mean.)

Know and know about

Know is followed by a direct object when the knowledge comes from direct personal experience. In other cases we use know about or know of.

I don’t know your mother. I have never seen her. (More natural than ‘I don’t know about your mother’)

We all know about Winston Churchill. (NOT We all know Winston Churchill) (Very few people among us have known Winston Churchill personally.)

Know and find out

When you know something you have learnt it. To talk about gaining knowledge, we use other words and expressions like find out, get to know, learn, hear and can tell.

‘She is getting married.’ ‘Where did you find that out?’ (NOT Where did you know that?)
He is from Boston and you can tell that from his accent.