When should you use an apostrophe?

An apostrophe is used to show possession.

Thus if the chair is mine, then it is Fred's chair.

Not Freds chair, or Freds' chair.

Note that with words ending in 'S' you may put at the end and omit the final S...

thus Jesus' followers is correct.

It is also used to show a missing letter.

Thus, it is OK to do this.
It's OK to do this.

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