In linguistics, the function of a sentence, or the sentence's function, is to announce the speaker's purpose for articulating the collection of words (being in sentence form, a fragment, a phrase, or a clause). There are four main different types of sentences: interrogative, declarative, imperative, and exclamatory.
The interrogative sentence asks a question. All interrogative questions end with a question mark.
This is the most common sentence. The declarative sentence is the one which simply states something. These sentences end with a period.
The imperative sentence is one which gives a command, direction, or a request. This type of sentence is used to get the listener, or audience of the sentence, to do something.This sentence ends in either a period or an exclamation point.
The exclamatory sentence is meant to express the strong emotion of the speaker. This type of sentence can be spoken to a listener/audience, or it can be spoken aloud to one's self. This type of sentence ends with an exclamation point.
In regards to the sentences above:
1. This sentence is an interrogative (based solely upon the question mark at the end). Outside of that, the fact that the words "must see" are capitalized, one could state that it exists as a exclamatory-interrogative hybrid. The words "must see" are capitalized for emphasis.
2. This sentence is an imperative. It is simply making a statement. Again, the curious part of the sentence is that it, also, has two specifically capitalized words. Therefore, emphasis is being placed upon a single part of the sentence. This makes the sentence an imperative sentence.