Regarding Sentence Formation?

I generally face problem in sentence structure .

Which one is correct :-

• Mainly he does this .
• He mainly does this .
• He does mainly this .
• He does this mainly .

All the above sentences seem to be correct and carry same meaning

I being a non native speaker of English generally face this problem .

Is there any rule regarding this problem which not only applies to this particular example but for all such similar examples ?

In a sentence more specific than 'He does this', the position of 'mainly' could affect the meaning.

(As his occupation) he mainly sells antiques (but also writes for the local newspaper).

(In his shop) he sells mainly antiques (but also modern ceramics).