19 Do not quench the Spirit.
20 Do not despise prophecies.
21 Test all things; hold fast what is good.
22 Abstain from every form of evil.
despise (v.20) = in Greek, an intensified form of the word for “reduce to nothing” — so, count as nothing, treat with utter contempt
prophecies (v.20) — the word means “make clear before,” which refers to that aspect of prophecy which tells of the future, but in Scripture, the word refers to the declaration of that which cannot be known by natural means, a forthtelling of a message from God, whether about the past, present or future. — Paul is instructing them not to write off revelation from God, whether directly communicated to him from God (as was the case with Paul himself) or whether written into Scripture (as was a the case with the letter from Paul they were reading at the moment, as well as all other Scripture).
With the completion of the canon of Scripture prophecy passed away (1 Corinthians 13:8-9). In his measure the teacher has taken the place of the prophet, (cp. the significant change in 2 Peter 2:1). The difference is that, whereas the message of the prophet was a direct revelation of the mind of God for the occasion, the message of the teacher is gathered from the completed revelation contained in the Scriptures. — Vine, page 92.
test (v.21) = prove, approve, examine
good (v.21) = kalos, intrinsically good (as opposed to agathos, that which is beneficial in its effect). Here, it refers to the quality of the teaching in itself
form (v.22) = lit. “that which can be see,” appearance, fashion, shape
evil (v.22) = poneros, malignant, bad in influence and effect (as opposed to kakos, base, bad in character)
form of evil (v.22) — The Greek can be interpreted as “every appearance of evil” or “every sort or type of evil.” There are plenty of other Scriptures that teach both meanings. It’s possible that Paul meant this to be understood in connection with revelation from God in the context of the previous three verses.