One word vs two words

Have you ever written sentences like these?

Enter your log in name
Login to the server

Verbs (words that convey an action) are sometimes combined with other words to make up what we call phrasal verbs. For example, 'log in' is a phrasal verb. You write phrasal verbs as separate words; not as one word. 

The single-word form (login) is actually a noun or describes a noun (adjective). Phrasal verbs usually have such a noun/adjective form, which is often a single word but is sometimes hyphenated. Writing the noun/adjective form as two separate words is just as incorrect as writing the verb as a single word.

So, the correct way to write the above sentences is,

Enter your login name
Log in to the server

In summary

1. If you convey an action, use separate words. 
2. If you convey a subject or describe a subject, use one word. 

Here are few more examples:

1. Did you print out the letter?
2. The printout of the letter is clear.
3. He forgot to shut down the server.
4. The shutdown was caused by a power failure.
5. This is a drop-down list.
6. Zoom in to get a detailed view of the image.
7. The zoomed-in view of the image is clear.
8. She walked up to the check-in counter to check in her bags.