The students were giggling in the classroom, so the teacher had to calm everyone down. is it a verb, gerund or participle

This is a verb. To explain this you need to understand the difference between verbs, participles, and gerunds. A verb is obviously an action that is being done to something. So the students are giggling. A gerund is in the form of a verb; however, a gerund acts like a noun. An example of this could be hiking. As a verb it would be, "We saw them last night, they were hiking up the mountain." As a gerund it would be, "When we go camping my favorite activity is hiking with Grandpa." The verb in that sentence is camping because you are doing an action and the gerund would be hiking because it is the name of the activity that you like the most. A participle is a word formed from a verb that acts as though it were an adjective. An example could be "Have you ever read The Giving Tree?" This would be a participle because giving is an action that you do;however, when you put it in that phrase it is describing the tree as a tree that gives.

The answer is a Complex sentence

Explanation:

Complex sentence is the type of sentence whereby there is atleast an independent clause and one or more dependent clause. since the independent clause can stand alone on its own, it makes it easer to separate from a dependent clause.

From the statement given above, The weather forecast has predicted severe thunderstoms is an independent clause which stood on its own. The dependent clause, the dark, heavy thunderclouds never materialzed is being linked together with an independent clause by a linking work but.