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1. The grammatical structure of an English sentence

In English, a sentence can be very simple:

but it can also be very complicated:

But they certainly have something in common that a sentence must have, and specific points to extend the meaning to the sentence.

We can easily see that the important "indispensable" component in a sentence is the verb. A sentence without verbs does not say anything.

For example:

Without verbs, we can not understand what is happening. John, coffee, in the kitchen, then what? When the verb is put in, the meaning of the sentence becomes clear:

Without verbs, we will not be able to describe anything. Therefore, verbs are a mandatory component in sentences.

We also know there must be something or someone doing this action, and that is the subject:

John is the one who does the action "made coffee in the kitchen", which means that John is the subject in the sentence. If you remove "John", the listener will not understand who "made coffee". Therefore, the subject is also an indispensable component of a sentence (*)

Here we have the "formula" for a simple sentence:

In the last example, we have identified two essential components in the sentence are the Subject (John) and the Verb (made). What is "coffee in the kitchen", and are they important?

To understand their need, let's remove it to see if the sentence still makes sense. First we remove "coffee" from the sentence:

When we see the sentence, the question we would ask is what John did. Without "coffee", the sentence becomes unclear because we do not know what John made.

"Coffee" is called the object of the verb. It is the word or phrase after the action verb to refer to the object affected by the subject. This is an important component in the sentence because it completes the meaning of the verb "made".

However, not all verbs need a new object, as in the first example sentence:

Therefore, we can say that the existence of the object in a sentence depends on whether the verb needs a object or not.

Next, we try to remove "in the kitchen":

This sentence is complete in terms of meaning. If we are not curious more about this action, we can understand what happened when we heard this: John made coffee, ended. Thus, it can be said that "in the kitchen" is a background information, as it works to clarify the meaning of the sentence more, to provide listeners to know where the action "made coffee" takes place but it is not a compulsory component in the sentence.

In fact, background information is not only limited to location information, but also includes many other things, such as time, manner, reason, etc.

These are the information that gives the listeners more data about the action in the sentence, so we call it "information background ".

If you still don't understand what background information is, try imagining a play: the character on the stage is the subject, the gestures and the character's actions are the verbs, and the scenery and props around the character on the stage is background information.

By analyzing the simple example so far, we can identify a sentence that includes the following components:

• Subject: is a person / object that performs action & is a mandatory component. (*)
• Verb: express actions in sentences & is a mandatory component.
• Object: is the person / act affected by the subject and may not be available, depending on the verb.
• Background information: adding other information related to the action, such as place, time, method, reason,... for the action to take place. It clarifies the meaning of the sentence more. It is not a compulsory component.

Since the subject and verb are two mandatory components in a sentence, so you must learn how to identify these two components. Once you have identified them, you will be aware of the remaining components easily.

(*) Sometimes we use imperative sentences in which a subject is not actually stated, but understood in the meaning.

Example:

YOU is the DOer of the action verb call. So YOU is the subject of the sentence.

When we are talking directly to someone, we use such statements and omit the word you. Therefore, the subject is YOU in statements like this one and it is understood in the sentence.

2. Determine the position of the verb

Before going into how to determine where the verb is in a sentence, you need to remember a GOLD PRINCIPLE as below:

So may be you have a big question: what is the main verb? Let's study more!

2.1. What is the main verb?

The main verb is always the verb that is signifying action within a given clause. For example, in the phrase "would have been running", running is the main verb, because it is the verb that signifies a real action of some kind. The others in the phrase are just helping verbs that enable to main verb to properly express its meaning. 

There can be more than one main verb within a sentence, depending on the complexity of the sentence. In general, there is only one main verb per clause; but if a sentence has multiple clauses and subjects, then each of those subjects could have its own main verb. Again, you just have to ask yourself: what action is taking place here?

Each main verb has five forms. Three of them can be used as a complete main verb:

The other two forms do not, by themselves, indicate tense:

They cannot be used alone as the main verb of a clause.
(Ann Raimes, How English Works: A Grammar Handbook With Readings. Cambridge University Press, 1998) 

2.2. Main Verbs in Verb Phrases

A verb phrase is the helping verb plus the main verb. The final word in a verb phrase, the main verb, carries the primary meaning of the verb phrase. Sometimes more than one helping verb accompanies the main verb. In the following sentences, the verb phrases are bold; HV appears [after] each helping verb, and MV appears [after] each main verb.

Notice that sometimes words not part of the verb phrase come between the helping verb and the main verb.

Typical helping verbs include: be, being, been, is, am, are, was, were, do, did, does, has, have, had, must, may, can, shall, will, might, could, would, should.
(The New York Times Guide to Essential Knowledge, 2nd ed. St. Martin's Press, 2007) 

2.3. Be, Have, and Do as Main Verbs

Several verbs, such as be, have, and do, have a variety of syntactic and lexical functions: They can be main or auxiliary verbs.

Every English sentence needs to have a verb to be grammatical. However, only the main verb is absolutely essential.
(Eli Hinkel, Teaching Academic ESL Writing: Practical Techniques in Vocabulary and Grammar. Lawrence Erlbaum, 2004)