There is a limit to the meaning you can express with the conjunction and. If someone says She has not been very successful and she is looking for another career, we can probably work out that looking for another career is a result of not being very successful, but sometimes joining two clauses by and can leave the connection to them very unclear:
The new CEO was appointed and Martin resigned.
This sentence describes two events but it doesn’t show what connection there was between them–always supposing there was any connection at all. We could link the two clauses in ways that did show a connection. For example:
Before the new CEO was appointed Martin resigned.
After the new CEO was appointed Martin resigned.
Although the new CEO was appointed Martin resigned.
The new CEO was appointed so Martin resigned.
and so on.
In compound sentences the clauses joined together are of equal status; we can cut the sentence up into clauses and each of them will become an independent simple sentence. Complex sentences work in a different way. One of the clauses is the main clause and the others are subordinate to it. The subordinate clauses form a single component of the main clause: subject, object, complement, or adverbial. In the first of each of the pairs of sentences that follow the subordinate clause is in bold type. In the second sentence it has been replaced by a word or short phrase.
 SUBJECT
What you did yesterday was inexcusable.
It was inexcusable.
 OBJECT
I cannot forgive what you did yesterday.
I cannot forgive your action.
 COMPLEMENT
That is what I admire about Billie.
That is it.
 ADVERBIAL
After the new CEO was appointed Martin resigned.
Afterwards Martin resigned.
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Multiple sentences are made by combining clauses. The simplest way of doing this is to use the grammatical equivalent of a plus sign:
She has not been very successful + she is looking for another career.
The commonest words to do this job are:
and but or
It is possible to string together as many clauses as you like in this way:
She has not been very successful and she is looking for another career, but so far she has not had much luck and has decided to try a different tack or even give up altogether, but . . . .
Until your readers either get lost or give up in disgust!
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If a sentence consists of one clause it is described as a simple sentence. Each of the following is a simple sentence:
Mary writes books.
She has not been very successful.
She is looking for another career.
If a sentence contains more than one clause it is described as a multiple sentence. Each of the following sentences is multiple:
She has not been very successful and She is looking for another career.
Although Mary writes books, She has not been very successful.
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More often an adverbial is not essential to the grammar of the clause it is in. In each case of the sentences that follow the adverbial is in bold type.
He found a secret passage behind the fireplace.
She is usually a very happy child.
Last week Mr and Mars Holt came to see us.
In each case if you remove the adverbial the clause is still grammatical and still makes sense. As the examples show, adverbials can occur in different positions in the clause.
Sometimes they can be moved to a different position:
He found a secret passage behind the fireplace.
Behind the fireplace he found a secret passage.
This does not mean that they can be placed anywhere in the clause. The following arrangement is impossible:
He behind the fireplace found a secret passage.
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There remains one clause component that is much less straightforward: adverbials. These are words or phrases that answer question such as:
 Where?
Here down the road
 When?
Later after a few hours
 How?
Slowly with great difficulty
 Why?
For the sake of the children because of the bad weather.
In a small number of clauses the adverbial is an essential component, just like a verb or an object; they are required by the type of verb used:
 subject+verb+adverbial
They have been living on their savings.
 subject+verb+object+adverbial
I placed the book on the table.
If you remove the adverbial from these clause you destroy their grammar and leave questions unanswered:
 They have been living …
 How?
 On their savings.
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