"Interjection" comes from the Latin words for "throw" ("ject") and "among" ("inter"). An interjection is thus a word or construction that is "thrown among" the words in a sentence. Unlike all the other words, interjections DO NOT chunk to the rest of the sentence. Instead, they usually indicate an attitude about the entire sentence. In speech, short interjections are common:

Interjections such as those above are not considered proper in formal writing, but the following, which consist of phrases, are common to both writing and speech:

"In fact" here simply emphasizes the writer’s belief that the sentence is factual, whereas "in my opinion" suggests that the sentence may not be. 

Subordinate Clauses as Interjections

     I like to consider the clauses in the following as interjections:

I have four reasons for doing this, one practical, the other three theoretical. In practice, it is much easier simply to put brackets around them:

We could, of course, analyze these two sentences as meaning:

To do so, students would have to rewrite the sentence (ugh). Nor do we want to put brackets around each part of the clauses:

To do that makes it appear that there are two subordinate clauses, when there are not.

      Theoretically, my preference can be justified by our model of how the mind processes language. Having dumped a main clause to long-term memory, short-term memory is cleared and the brain will take whatever it finds that can be a main S/V pattern as a main S/V pattern. (which is why subordinate clauses at the beginning of a sentence must have a subordinate conjunction). In our example, the brain would take "It was" as a main S/V pattern. It must then handle the "I think" (or "he said") as a subordinate clause.

      Another theoretical reason is that such clauses fit the definition of an interjection: they indicate the speaker/writer's attitude or comment about the sentence as a whole. My final theoretical reason is based on a hypothesis, a hypothesis that can be confirmed by more statistical research. The hypothesis is that written interjections appear late in natural syntactic development. A seventh grader is likely to write "I think it was a big mistake." A college graduate is more likely to write "It was, I think, a big mistake." If, in analyzing and marking sentences, we count them both as "I think [it was a big mistake], we would have to reanalyze everything just to distinguish one version from the other.
     When clauses are used as interjections, they are often set off either by dashes or parentheses: