grammar - correct use of hyphens to connect two words

Should I use a hyphen to connect the following words? What is the general rule about that?

environment friendly --> environment-friendly

frontend --> front-end

backend --> back-end

speedup --> speed-up

Answer

Two words are joined by a hyphen when the collocation occurs in a non-standard context. For instance, we speak of the front end (space, no hyphen) of a car when this acts a noun phrase:

The front end of my Chrysler was dented.
My Chrysler was hit on the front end.

But when we use front end as an attributive, we hyphenate it:

My Chrysler suffered a front-end collision.

This lets the reader understand that we are speaking of a collision involving the front end rather than an "end collision" (whatever that might be - it has no obvious meaning) at the front.

In the same way, the phrasal verb is always speed up (space, no hyphen) when we use it as a verb:

Whenever we're behind schedule we speed up.

But if we want to use speed up as a noun, we hyphenate it.

We were behind schedule, so the foreman ordered a speed-up.

Two words are compounded - stuck together with neither a hyphen nor a space - when the collocation acquires a distinct sense, or the phrase becomes so common that it is felt to be a single word rather than two separate words. For instance, a dead line or dead-line was originally a line around a military prison which a prisoner could be shot for crossing; but when the phrase came to be applied metaphorically to the point by which a project must be completed, and became very common in the sense, it turned into the single word deadline.