Learn English – the difference between “will not” and “will never”

Fill means to add content to the container or gap until it is full.

In particular, "filling" tends to involve a physical action, such as filling a mug with water, or filling a form in with a pencil.

Please fill this jug with water

Please fill in this form.

I have filled in all of the empty days on the schedule with stuff to do.

We need to fill the hole in the road with new gravel

We need to fill the vacancy in sales before Tuesday.

Fulfil on the other hand means to complete an action or requirement. In particular, one tends to fulfil metaphorical or non-tangible things, such as requirements, objectives or promise.

You need to fulfil all of the requirements before the end of January.

Fulfilment of the key objectives is our primary goal.

I'll fulfil my promise to help you with your homework before it's due in, really I will!

The fourth one is correct "I have never learned it"

The first is the present form, though it's not often used, in fact I can't think when I would say "I never learn it"

The second if not correct grammar

The third is a correct past perfect form, "I never learned it" and you could use this ad well. You might use it if you were explaining for example that you were never taught how to drive, so you might say "I never learnt how to drive". But if you are saying I was never taught French you might say "I have never learned French" I think you can use these forms interchangeably.