well, I use that phrase so frequently (notes for work) that I need to be sure I am doing it correctly...Naive speakers don't usually say "assist someone to do something". If you do that in school, a teacher will probably correct you. However, it is understandable, and it's likely that some native speaker somewhere has used it.
Thank you !, the matter is that I should use the verb assist, not help. And yes, is for using it in formal writing.To sound fluent in English, you can’t go far wrong if you use the verb help rather than assist in all everyday spoken contexts.
Assist is mainly used in formal writing (e.g. “Governments have a responsibility to assist refugees”), or in other forms, such as “Can I be of any assistance?”.
I helped him to knock down his garden wall.
Note that when you assist/help someone, it may be important to differentiate between:
--- working in partnership with the other person - eg you both worked together on the task;
--- doing the task FOR the other person;
eg I helped Jack by knocking down his garden wall for him while he was in hospital.
I helped him to fill in the form. He did most of it himself, but some questions were quite difficult.
I helped him by filling in the form for him.