Fill up
To fill up means filling up something that is empty with something else, like filling up a glass with water.
If you are referring to a form, you fill in a form (British English) or fill out a form (American English).
Grammar
Too many people still spell this word as grammer.
There is no such word as grammer – it's grammar.
In order to
This is an old-fashioned wordy phrase that can easily be shortened by removing the first two words.
Just say to.
Learnt/Learned
Both of these words are correct as the past tense and past participle of the verb learn.
Learnt is more common in British English.
Learned is more common in American English.
Mention
Mention and mentioned are not followed by the word about (same as discuss and discussed).
For example:
The teacher mentioned that next Monday is a public holiday.
The last thing I want to mention today is the importance of email etiquette.