Preferably all the time, it makes communication so much easier when your audience is general, rather than just a small group of friends. Especially when applying for jobs. And these three sentences are examples of poor grammar.
If you are writing proper grammar you should not use 'then' at the beginning of a sentence.
It's you are, you is is not proper grammar.
It's you are, you is is not proper grammar.
Use proper grammar!
Nope!
"She did not have" is the proper grammar.
very good grammar (proper grammar)
Use proper grammar
Yes, but you sure didn't.
Use proper grammar.
if you want a proper answer use proper grammar
if you want a proper answer use proper grammar