mainly
[adverb]
Synonyms: chiefly, for the most part, in the main, largely, mostly, on the whole, predominantly, primarily, principally

[TahlilGaran] English Synonym Dictionary ▲

main‧ly S2 W2 /ˈmeɪnli/ adverb
used to mention the main part or cause of something, the main reason for something etc Synonym : primarily:
Her illness was caused mainly by stress.
The workforce is mainly made up of women.
I don’t go out much, mainly because I have to look after the kids.
Increased sales during the summer were mainly due to tourism.
We talked about various things – work, mainly.
GRAMMAR
Mainly is not used at the beginning of a sentence:
They play mainly on Wednesday evenings (NOT Mainly they play...).

REGISTER
In written English, people often prefer to use chiefly or primarily, which sound more formal than mainly:
The disease is caused chiefly by poor sanitation.

[TahlilGaran] Dictionary of Contemporary English ▲

mainly
adverb
1.
BAD: We were attracted by the clean air, the beautiful scenery, and mainly the peace and quiet.
GOOD: We were attracted by the clean air, the beautiful scenery, and above all the peace and quiet.
BAD: What I like about her mainly is her sense of humour.
GOOD: What I like about her above all is her sense of humour.

Usage Note:
To show that one particular feature or item is more important than all the others, use above all : 'The person we're looking for has got to be smart, intelligent and above all trustworthy.' 'Above all, the government wants to avoid an increase in inflation.'

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2.
BAD: Mainly the film is about the madness of war.
GOOD: The film is mainly about the madness of war.
BAD: Mainly these problems involve the definition of words.
GOOD: These problems mainly involve the definition of words.

[TahlilGaran] Dictionary of Common Errors ▲