Before the noun: hyphenate
a well-known writer, a part-time job, a 10-year-old girl.
After the noun: usually not
The writer is well known. The job is part time. The words no longer work as a unit modifying a noun that follows.
No hyphen after -ly adverbs
a highly skilled team, a newly built house. The -ly already tells the reader it modifies the next word.
Prefixes
Most prefixes close up (reelect, nonprofit, preheat). Use a hyphen before a capital (pre-Victorian), with self-, ex-, all- (self-aware, ex-partner), and to avoid misreading (re-cover a sofa vs recover from illness).
Numbers
Hyphenate compound numbers from twenty-one to ninety-nine and fractions used as adjectives (a two-thirds majority).
Frequently asked
Is it "e-mail" or "email"?
Most current style guides (AP, Chicago, Merriam-Webster) use email.
Do you hyphenate "long term"?
Before a noun, yes: "a long-term plan". After, no: "planning for the long term".