Boys loves to cry. She didn't went to the market. They are going for the party, isn't it?
Is this your homework assignment?
*'Boys loves to cry.'
*'She didn't went to the market.'
A Generativist model would probably explain this as 'do' occupying the spot where tense normally is licensed.
However, there are languages where double tense marking is grammatical (and also double person agreement; this is even found in one Indo-European Language: Bulgarian).
'They are going for the party, isn't it?'
Actually this is grammatical.
In some dialects of English in the UK this is grammatical. In many dialects, the clitic innit can be used even when referring to plural subjects, and even to personal pronouns. 'Isn't it' has been grammaticalised as a negative tag question:
'Dogs are in the room, innit?'
'You like pizza, innit?'
Essentially, 'isn't it' can be used to mean:
- isn't it/aren't they/aren't we/aren't I/aren't you/aren't those/aren't these
- don't you/don't I/don't they/don't we
Of course the two above sentences would both be highly ungrammatical in any American, Australian, New Zealand, and Canadian English dialects.