None of the above applies to simple phrases. You can disregard them utterly even if you have never published a word before.

And, of course, it is perfectly possible to break rules without knowing them, and still be understood - and that's what most writers are aiming for. After all, what authority is there for any rule of grammar? Certainly not OUP or the US Government Printing Office. Their standards have persuasive authority only.

Common usage is the only reliable authority I can think of.

I, personally, reject any notion that the possesive form of Jesus is Jesus' and the suggestion that the possessive of conscience is conscience' is supremely ridiculous.

It's not bad grammar that is the problem - any one of us can cope with grammatical mistakes while we are reading, and most of us will pass over them without even noticing. The rest of us are probably pedants.

No, the real problem is poor expression: not making the proper connections between your thoughts and your writing. You know what you mean, but somehow, you set it down on paper ambiguously.

I once submited a piece of writing for a competition in which I broke virtually every rule of grammar that applied. But my story made perfect sense and conveyed the flavour of the character's thoughts that rigid grammatical precision would have destroyed. I didn't think, as I was writing, Ah! I shall ignore the need to make sure subject and verb agree. I simply had to write it that way.