"two year period" or "two years period"

[MonaDeme]

Dear all,

I am trying to understand the difference between "two year period" and "two years period" or "tow-year period". I am not sure I understand when to use one and when to use the other one or which one is correct/wrong.
So far, to what I saw on the internet "two year" it should be a compound words therefore, it should be written using a hyphen "two-year" or is it not the case? However, I thought that when we are talking about many years we use the plural form. This is the part which is not clear to me why it should not be "two years period" or maybe "two-years period" instead of "two year period"? Is this because I am talking about one period of time of 2-year/-s?

(Here is the sentence I was talking about:
A longitudinal study was conducted over a two year period.)

A second question, is there a rule when two-year is a compound word and when not? When do I need to use the the hyphen and when not?
For example if I want to say: I met him two years ago. Is it correct to write without hyphen or with?

Any comments are highly appreciated.

Thank you!

 

[Tdol]

It should be a two-year period. As it is functioning as an adjective, it doesn't take the plural.

 

[emsr2d2]

Dear all,

A second question, is there a rule when two-year is a compound word and when not? When do I need to use the the hyphen and when not?
For example if I want to say: I met him two years ago. Is it correct to write without hyphen or with?

Any comments are highly appreciated.

Thank you!

With regard to your final question, if you think about it like this, it might help:

Saying "I met him two years ago" is the same as "I have five bicycles now".

A verb + number + noun + time period.

I'm sure you wouldn't think it was "I have five-bicycles now" so, by the same token, there would be no reason to hyphenate "two years".

If, however, you said "I got to know him over a two-year period" then you need the hyphen because "two-year" is a single adjective modifying "period".

Be careful though, because you could also say "I got to know him over the space of two years."

 

[kieuthuyvan]

I think it can be either "a two-year period" or "two years' period"

 

[Chrystalline]

I think it can be either "a two-year period" or "two years' period"

No, Tdol and emsr2d2 are correct. It's "a two-year period" because "two-year" is the adjective describing "period." When we're talking about time, a single "period" can be any length:

a thirty-second period
a five-minute period
a six-day period
a nine-week period
a seven-month period
a two-year period
a three-century period
a fifty-million-year period

As emsr2d2 demonstrated before, the hyphen is not included if it is not an adjective:

She met him thirty seconds ago.
We enjoyed five minutes together.
I got to know him over the last six days.
You have class for nine weeks.
Everyone has to work for the next seven months.
He has been here for two years.
The conflict has lasted almost three centuries.
They said this fossil dates back fifty million years.