To know something is to have learnt it, not to learn it. Note that we ‘learn’ before we ‘know’. Know is used when learning is finished.

To talk about getting knowledge we can use words like find out or learn.

I want to learn English. (NOT I want to know English.)

I know him. (NOT I learn him.)

Last year I learnt to swim. (NOT Last year I knew to swim.)

He went to school to learn algebra. (NOT He went to school to know algebra.)

Later on he realized his mistake. (NOT Later on he knew his mistake.)

Learn can be followed directly by an infinitive. A structure with learn how to is also possible. Usually there is no difference of meaning.

He soon learnt to use a computer. OR He soon learnt how to use a computer.

It is not easy to learn to drive in a city. OR It is not easy to learn how to drive in a city.

Learn is an irregular verb in British English. The three forms are: learn / learnt / learnt.

In American English, learn is regular. The three forms are: learn / learned / learned.

She learned some French when she was in France. (US)

She learnt some French when she was in France. (GB)