简直【jiǎnzhí】

简直 goes before the verb or adjective and pushes a statement to the edge: the speaker knows it's an exaggeration and means it as one. It belongs to speech and lively writing.

What follows it

What comes after 简直 has to be strong: 太…了, a 不 + verb of disbelief, or a comparison with 像…一样 or 是.

A mild predicate sounds odd after it.

• × 今天简直有点儿冷 → 今天简直冷死了

简直 or 简单

The two share 简, and learners mix them. 简单 is the adjective "simple, easy"; 简直 is an adverb and never means "easy".

• × 这个问题很简直 → 这个问题很简单