In informal and formal speech or writing, people use God's.
God's love is eternal.
No, you do not need to use an apostrophe in "athletes" when referring to the plural form of the word. An apostrophe is only necessary to indicate possession (e.g., "the athletes' performance"). In general usage, simply writing "athletes" is correct.
It is correct grammar to use "God" if the word is assigned as a name or a proper noun. The word "god" is used when referring to a generic deity.
If you are referring to possession, use the apostrophe: "The TV's screen was cracked." If you are referring to multiple TVs, use the plural form without an apostrophe: "There were two TVs in the living room."
Both could be correct, depending on the context. The capitilized proper noun God is used to refer to the Christian and Jewish God. So you wouldn't use the capitalized form without an apostrophe because it makes it plural which doesn't happen with most proper nouns. You could only do this if you were using the common noun 'gods' which has a completely different meaning. Probably you are wanting to use the word in the possessive sense - referring to something that God is or has ...such as God's mercy, or God's creation. You must always use the apostrophe and then s with this word. ( There are other words where this rule doesn't apply....but that's another English lesson...and doesn't apply to the word God.)
yes
The correct spelling is apostrophe.
Unless it means "it is", there is no apostrophe in "its". See related question.
anal insert
Yes, you would use an apostrophe in "dinner at the Teodoros'" if referring to dinner at the home of the Teodoro family, indicating possession. The apostrophe comes after the "s" because "Teodoro" is a plural surname. If referring to just one Teodoro, it would be "dinner at the Teodoro's."
Yep :)
A possessive pronoun does not take an apostrophe. This is an exception to the rule that an apostrophe indicates the possessive. To write, 'the dog lost it's bone,' is not correct. The correct way is without the apostrophe: 'The dog lost its bone.' 'It's' (i.e., with an apostrophe) is correct only when used as a contraction of 'it is.'
Yes, there can be either apostrophe s ('s) or just an apostrophe (') at the end of the word.