Know | Know How To

• I know how to make French fries.

In a formal style, know is occasionally followed by object + infinitive.

• I knew him to be an honest man.

However, this is unusual; that-clauses are generally more natural.

• I knew that he was an honest man.

Tenses

Know cannot normally be used in the progressive form.

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

Know and know about/of

Know + object is used mainly to talk about knowledge that comes through direct personal experience. In other cases, we normally use know about/of, have heard of or another structure.

Compare:

• ‘Do you know my brother?’ ‘No, I have never met him.’
• We all know about Albert Einstein. (NOT We all know Albert Einstein.)

I know and I know it

I know refers to facts – it could be completed by a that-clause.

• ‘You are late.’ ‘I know. ’ (= I know that I am late.)

I know it refers to things – it replaces a noun.

• ‘We went to a nice restaurant called Fire and Ice last night.’ ‘I know it. ’ (= I have heard of that restaurant.)
• ‘We went to a nice restaurant called Fire and Ice last night.’ ‘I know.’ (=I know that you went to that restaurant.)