Yesterday by the Beatles

[Notorious]

Hello Folks, I'm so happy i've got a minimum level of knowledge so that I can try to ask your help on the subject I'm very interested in.
All my life I've been thinking of what kind of English grammar rule is used in the following sentence. "Now it looks as though they here to stay" What Paul McCartney put by this expression? He meant:
1. It looks as though they (troubles) have stayed here
2. It looks as though they keep staying here
3. It looks as though they're going to stay here

I would be glad If someone could explain me what kind of rule is it and what kind of cases it should be used in.

[shannico]

I thought it was
as though they are here to stay...(the troubles of course), meaning it looks as though they won't go or leave me alone.
That has always been my understanding of it.
Great song!!!

[BobSmith]

You've slightly misquoted the lyrics:

Yesterday, all my troubles seemed so far away
Now it looks as though they're here to stay

[Raymott]

Hello Folks, I'm so happy i've got a minimum level of knowledge so that I can try to ask your help on the subject I'm very interested in.
All my life I've been thinking of what kind of English grammar rule is used in the following sentence. "Now it looks as though they here to stay" What Paul McCartney put by this expression? He meant:
1. It looks as though they (troubles) have stayed here
2. It looks as though they keep staying here
3. It looks as though they're going to stay here

I would be glad If someone could explain me what kind of rule is it and what kind of cases it should be used in.

This means 3. It's not just about the present or past. It is also (perhaps mainly) about the future.
"Here to stay" is an idiom. If something is "here to stay", you can also say it's "here for good" or "here forever".

[Tdol]

I agree- it's 3.

Also, Paul McCartney writes songs and will put grammar second to the requirements of the tune, so don't expect perfect English all the time from him.

[Notorious]

I wholeheartedly thank you all guys for leaving the comments. Now I do know the idiom that Paul used.