Two-word verbs described with One-word nouns

Question

I've noticed that certain (compound?) verbs are combined into one word when the process is used as a noun. It seems to generally be processes with a preposition in them. If the noun isn't combined into one word, it's usually at least hyphenated, which seems to be a pretty standard English word progression (two words, hyphen, one word).

Is there a name for this phenomenon? Do we know why it happens? Do the verbs stay separate or will they eventually combine like the nouns?

Examples:

• I log in using the login form.

• You can make up for the missed test with the makeup quiz.

• From this lookout tower, you can look out for miles.

Answer 1

There are two different cases: phrasal verbs and prepositional verbs, which can be a bit subtle to tell apart.

With phrasal verbs, a direct object can come between the verb and preposition:

I took the ladder down.

With prepositional verbs, a direct object cannot come between:

*I came the ladder down.

If the direct object is a pronoun, it must go between:

I took it down.
*I took down it.

This separability is probably why the phrasal verbs typically stayed spaced (although closing up the verb+preposition is certainly seen, many would consider it an error).

However, some phrasal and prepositional verbs have related nouns: There is such a thing as a "comedown" (a disappointment), and a "takedown" (in wrestling). As nouns, those are no longer a verb plus a preposition, and they get closed up (no space, or sometimes a hyphen).