Conjunctions are an important method of extending sentence length and complexity, because they are a common method of joining words or parts of sentences together.  Coordinating conjunctions join independent clauses (as well as words and phrases), while subordinating conjunctions can join both dependent and independent clauses (as well as words and phrases).

The acquisition and frequency of conjunctions have both been studied extensively.  Among the findings are that the word and often initially takes the role of other conjunctions  (Bloom et al., 1980; Scott, 1988; cited by Owens, 1996).  The conjunctions but, so, or, and if soon are acquired in typically developing children to serve functions that and isn’t as easily able to achieve.  Conjunctions like because then develop to express not only a relationship between sentence elements, but additionally a temporal sequence.  According to one estimate, by the time a normal child’s mean length of utterances reach 5.0 (at an average age of 4 to 5 years), 20% of the sentences they use in spontaneous speech contain embedded or conjoined clauses (Paul, 1981).

Language itself doesn’t require conjunctions, but effectively communicating advanced ideas usually does.  As with other language modalities, conjunctions exist because they assist.  We use them to achieve a goal.  Just try giving a reason for something without using the word because, or try describing the time relationship between two completed events without using conjunctions such as before, after, or then.  It can be done, but much less effectively.

Generally, developmental order of conjunctions is determined by the complexity of the relationship the conjunction serves.  Conjunctions appear frequently in assessments such as the CELF, CASL, OWLS, and SPELT.  Also, Conjunction Junction is a timeless piece of art.