Adverb
If "get along with" is considered a small enough number of words to have a part of speech as a phrase, it is a verb.
"To get along" is an infinitive phrase.
"a" is an article along with "an" and "the"
The word "along" can function as both an adverb and a preposition.
In the case it is an adverb, as in accompanying or together with, for example 'consider the advantages along with the disadvantages' because
"Along" is an adverb, or sometimes a preposition, depending upon how it is used in a sentence.
If "get along with" is considered a small enough number of words to have a part of speech as a phrase, it is a verb.
Black and white got along after Martin Luther King Jr. made a speech
on, under, near, along, beneath, above, with, and so on.
Spontaneous language is a term used in linguistics, psycholinguistics, speech pathology (and perhaps other areas) which refers to speech that is not parroted but an original expression or construction.
"Backwards" is an adverb: "The car moved slowly backwards along the street."
Which, along with who, that, whose, what, how, etc., are relative pronouns. They can also be used as subordinate conjunctions.