Hello everyone!
Which one is better?
We can also use an infinitive in some expressions when we want to emphasize something.
or
We can also use an infinitive in some expressions when emphasizing something.
These sentences are too similar. I would use shorter version.
why not emphasise your point by using an infinitive… 
“Infinitives can be used to emphasise something.”
(or if you want it even shorter, and as ‘something’ is a bit weak - “Infinitives can be used to give emphasis”)