If I say

The first *groups presentation was on insects.

is it group's or groups'?

closed as off-topic by tchrist, choster, Edwin Ashworth, anongoodnurse, FumbleFingers May 9 '14 at 11:49

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  • If you can say "The presentation of the first group" instead, then it is possessive, with an apostrophe. Possessives always take an apostrophe on nouns; it's only pronouns that cause trouble. – John Lawler May 6 '14 at 3:22
  • Also see: English Language Learners – Kris May 6 '14 at 5:22
  • Do you mean "a presentation by a particular group"? Or do you mean "a series of presentations called groups"? – Matt E. Эллен May 9 '14 at 10:00

There can only be one The first group, so singular. The first group's -- singular with an apostrophe-s.

The one exception would be where you intend to say the first (few) groups. However, there would then be ambiguity. One would better avoid first with a plural as in that case.

The first group's presentation was on insects.

As Prof. Lawler observed, it is better to use the of form in formal writing instead of the apostrophe in such cases.

The presentation of the first group was on insects.

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