We need to be careful to distinguish between the kinds of "units" that we produce in writing, especially formal academic writing, and the kinds of "units" that we produce in conversational speech. Sentences in academic writing are usually syntactically "complete" units with full subjects and predicates. On the other hand, conversational speech proceeds with little bits and pieces of language--few people speak in full sentences. Some grammarians and linguists use the term utterance to refer to the units of spoken conversation.They've invented this term as a way to clarify the unit being studied and as a recognition that while both speech and writing use the same basic grammar they are not completely alike in how they use that grammar. 

For us as ESL/EFL teachers, the distinction between sentence and utterance is important as a reminder that we don't want to teach our students to speak in "sentences." We need to allow for the appropriate units in the appropriate kind of communication. That's why saying "answer in a complete sentence" is such a strange requirement in a spoken grammar activity.

You might notice in reading grammar reference materials that grammarians and linguists prefer the term clause to sentence. Of course specialists in any field of study do have an urge to create our own inner-circle vocabulary, using jargon to mark insiders from outsiders. On the other hand, widely used words like sentence can be so vague that they are difficult to use for precise study. 

Grammarians use clause to refer to both simple sentences and to subordinate clauses. A simple sentence is called an independent clause. Some of the definitions of clause used in the Longman Grammar are quoted here:
 
 

The term simple sentence can be confusing because of definitions that many of us have learned that focused on the meaning or content of a sentence. Those misleading definitions said something like "a simple sentence has only 1 main idea." But that definition is just impossible to apply because it's impossible to be sure what "1 idea" is. A simple sentence can be short and with uncomplicated ideas--but a simple sentence can be long and with complicated ideas. 
 

Thus, we need to use a definition like the one given in the Longman Grammar. A simple sentence is a unit that has only a single subject and a single predicate. The subject can be compound, but the simple sentence is a single unit as in the following examples. 
 
 

Reference grammars usually definite sentences in terms of verb types. This analysis leads them to recognize 7 varieties in the category "simple sentence"--each based on a type of verb. There's one simple sentence type that involves intransitive verbs--their S+V. Linking verbs lead to two simple sentence types: S+V+SP and S+V+A. Transitive verbs lead to 4 types--because of the 4 possibilities for different types of objects in their predicates: S+V+DO, S+V+IO+DO, S+V+O+A, S+V+DO+OP.

Before we consider simple sentences in context, let's go through these seven types to be sure that you understand how they differ. 

Intransitive verbs cannot have objects or complements. They are complete with just a subject and a verb. Adverbials can be added but are not required for the SV to be syntactically complete.
 

I've taught so many ESL students over the years and sometimes-less-than-mature graduate students, too, that I've just learned not to try to talk about copular verbs. It's just not worth the giggles. That's probably why classroom teachers still use the term linking verbs. Moreover, linking does have the right meaning for this category. The verb links descriptive information back to the subject. Here's the list of common linking verbs from Applied English Grammar:

Linking verbs can connect adjectives or noun phrases to give more information about the subject of the sentence. To distinguish this purpose and structure from that of the object in the SVO types, some linguists and grammarians use the term complement for the third element in a sentence with a linking verb. Others use the term subject predicative.  

Two types of sentences with linking verbs are given in the Longman Student Grammar: S+V+SP has an adjective or a noun as the subject predicative. Notice that most linking verbs can take only adjectives for their complements but that be can have either an adjective or a noun phrase. The second type is the S+V+A where the A is required and is often a prepositional phrase used for location or time.
 

Simple sentences are important in grammatical analysis because they are used as the foundation block for explaining other structures: complex and compound sentences are explained in terms of simple sentences (or in terms of independent clauses). Question types and formation are explained in terms of changes made to simple sentences. 

But simple sentences in context are another matter. What kinds of sentences do ESL/EFL writers produce? How do proficient writers use simple sentences? 

Some research has shown that moving from writing simple to complex sentences is a developmental pattern for ESL writers. Initially, lower proficiency students write mostly simple sentences--with a single subject and predicate. As their vocabulary expands and their knowledge of English grammar develops, ESL writers start to use more complex sentences. The change, of course, also reflects instruction; ESL writers are learning to use the types of sentences that their teachers require them to use and that they find in the materials that they are reading.

Discourse studies that have analyzed academic text show that most of the sentences in formal academic writing are complex sentences with dependent clauses of various sorts. Simple and compound sentences are used much more rarely and often for specific purposes. For example, some writers use short simple sentences at the end of passages to punch home a point in a dramatic way through the contrast in length. In the sociology textbook that I'm using for examples in this course, the writer has a pattern of starting paragraphs with short simple sentences and then adding details and development with more complex sentences. He does not always follow this pattern, but it recurs often enough to be a regular feature of his text. Here are four beginnings of paragraphs that show how he is working with sentence types in his presentation of complicated content:

 

As ESL/EFL teachers, we need to know more about the developmental stages that we are helping our students to move through, giving students opportunities to read (and analyze) writing that uses complex sentences with various sorts of subordinate clauses and other opportunities to learn about those structures and how to use them for their own writing. However, we also need to know more about how advanced, proficient writers use various sentence types: simple sentences are not dropped from a writer's style sheet as we become more advanced but seem to be relegated to special uses. Thus, our goal isn't to get students to stop using simple sentences but rather to learn to use an effective mixture of simple, compound, and complex sentences appropriate to the type of material that they are writing. 

That sounds great, doesn't it? How do we do it? I'm interested to know more about your ideas and experience in teaching students to work with English sentences.

Please email me your questions and comments. Thanks.

        Biber, D., Johansson, S., Leech, G., Conrad, S., & Finegan, E. (1999). Longman grammar of spoken and written English. Edinburgh Gate, Harlow, England: Pearson Education Limited.

        Celce-Murcia, M. & Larsen-Freeman, D. (1999). The grammar book: An ESL/EFL teacher's course.  (2nd ed.)  Boston: Heinle & Heinle.