Apostrophes are a real stumbling block for many people, as are alternative such as “lying” or “laying.” There is no need to panic because taking a look at the rationale behind different alternative can help you to choose the right one for the sentence.
Apple's or Apples?
An apostrophe either shows that letters are missing, or that something belongs to something else. They are never used just for plurals. Even when putting an "s" after acronyms to make them plural you should never use an apostrophe. Just write "MPs" or "CEOs" and let the difference between capital and small letters make your meaning clear. If you think it would be really confusing, expand the acronym, e.g. "Members of Parliament", "Chief Executive Officers".
The apostrophe for possession always comes straight after the thing or person which has possession of something else. If you are confused about where exactly to put the apostrophe with plurals remember the trick of changing it for "belonging to".
For example, if a child needs a signature from one parent for a school event then he needs his "parent's signature" - the signature belonging to the parent. If he needs the signature of both parents then he needs his "parents'signature". The apostrophe comes after the "s" because he needs the signature belonging to his parents.
In the same way you may have hated "last week's meeting" - the meeting that belonged to (took place in) last week, but you would ask for "two weeks' wages in advance" - the wages belonging to the following two weeks.
It is the same with names ending in "s". Something belonging to James is James', not Jame's (because that would mean it belonged to Jame, not James). With names ending in "s" it is acceptable to put another "s" after the apostrophe (James's) but that is up to personal taste.
Lay or Lie?
Another one that confuses people is whether they should use "lay" or "lie" for the act of reclining. One reason for the difficulty is that "lay" is the past tense of "lie". You can say "I lay down last night" but can you say "I will lay down just now"? The answer is no. In the present and future tenses "lie" is something you do yourself but "lay" is something you do to something else (or someone else does to you).
For example you can lay a table, because that is something you do to the table. And your lover can lay you down on the bed, because that is something he or she does to you. But you yourself can only lie down on the bed, not lay.
In the same way, a thing cannot simply "lay", because this is an act you can only do to something else. So you can say "The dog was just lying there." but not "The dog was just laying there." When in doubt ask yourself "Am I just doing it (lie) or am I doing it to something or someone else (lay)?
I Was Sat or I Was Sitting?
This one is very similar to the "lay" or "lie" problem. You can say "I sat in that chair" but can you say "I was sat in that chair"? Again, the answer is no. Although it is correct in some dialects and has become widespread, "was sat" is a mistake if you are talking about something you just did yourself. If somebody had picked up up and sat you in the chair then you were sat, but if you did it yourself the correct phrase is "was sitting".
To remember it, compare it to the present tense: You would say "I sit in this chair" so you also say "I was sitting in this chair". "I was sat" and "I am sat" apply only to cases where you were physically moved by someone else - a pretty rare occurrence for most adults!
