Q&A for How to Use "That" and "Which" Correctly

"Which" is used in a question when you are referring choices or options. Some examples would be "Which flower do you like?" "Which song do you want to play?" "Which store are we going to?"

"Should" is a suggestion; "must" is an insistence, usually used when something is imperative.

Almost. "...that I could not go to class."

Please see the attached document, which my boss signed.

The "5 W's" (what, where, when, who, and why) are the five questions considered important in gathering and reporting factual information. ("How" is sometimes included in that list.) "Which" is not typically included.

Yes, that's what it means. The sentence essentially says "I like red flowers." "That are red" is a restrictive clause (it helps to define the meaning of the sentence), and so the word "that" is required.

This depends on the level of formality you want. In the first sentence, the word "that" is understood and is often left out in speech. The second sentence is technically correct; however, you might also consider simply saying, "I chose a gift to give to her" or "I chose a gift for her."

Yes, that's correct.

In this sentence you would not have to use either word: "...the watch my father bought..." is perfectly acceptable. However, if you want to insert one of those words, make it "that," because "my father bought for me" is a restrictive clause identifying a specific watch (rather than merely adding extra, non-essential information).

Use "that." "That I gave you" is a restrictive clause: it identifies a particular pen and tightens the meaning of the sentence. You could also say, "The pen I gave you is red."

Yes.

It depends on the intention of the sentence. If it merely adds an incidental reference to Baroque works, it's a non-restrictive clause introduced by "which." If the clause is restrictive and points to a specific organization distinguished by its Baroque presentations, it would be introduced by "that."

"That" would be better, because the clause "you gave me" identifies a specific book and does not merely add inessential information. Alternatively you could say "the book you gave me is very interesting."

Those phrases are a quick way of adding information without having to rename the subject of the information.