grammar problem

They are planning to get married in the summer

我想問下拉,married -<係adj黎,咁get係verb ,點解後面唔係用adv,而會用adj?或是get married係1個phrases係一定要咁用呢?

My explanation:
get = linking verb 繫動詞, 其後一般常跟past participle (在這句式的past participle is used as adjective ); get 後的過去分詞常作形容詞或用作被動式。
**linking verbs 繫動詞後不用副詞,用形容詞
seem/ look/ appear/ get 這些繫動詞, 後面都用adj not adverb

問: 是get married係1個phrases係一定要咁用呢?
答: no
sentence structure句式= personal subject 人 + get + past participle過去分詞
= phrasal expression = to cause oneself to become 使自己/本身成為
get married = (他倆人自己/本身) 成婚
e.g. They must have got lost. 他們 (自己) 一定是迷了路。
They have just got married.他倆人剛成婚。
Tom got punished. 湯姆被罰(受罰) = (被動句式)

They are planning to get married in the summer.

question: "married" is an adjective; why does it appear after a verb "get"?

That is because "get" functions as a linking verb here in your sentence.

get = [linking verb]to change to a new feeling, situation, or state [= become] (Longman dictionary)
example: Don't get upset.
Eat your dinner before it gets cold.

It is grammatical to have [linking verb] + [predicative adjective].