Connectors are used to show a relationship between two sentences or ideas.

Look at this two paragraphs:

The library on 5th Avenue in New York City is one of the best places to do research.  It has hundreds of the most respected magazines and journals in the world.  

The library on 5th Avenue in New York City is one of the best places to do research.  Moreover, it has hundreds of the most respected magazines and journals in the world.  

We can use connectors to:

Express addition

Moreover, furthermore, in addition, besides, what’s more ⇒ followed by a comma after a pause.

As well as, in addition to ⇒ add an extra piece of information, with noun phrases.

I didn’t want to go shopping. Besides, I had no money left.

We bought the cat. What’s more, we bought some toys for it.

I don’t want to talk about this. Moreover, she hasn’t called me.

Expressing contrast

However, nevertheless, still, yet, even so, on the contrary, in contrast ⇒ followed by a comma, introducing an idea that contrasts the previous one.

Although, (even) though  ⇒ link two contrasting ideas.

In spite of, despite, in contrast to, contrary to  ⇒ link two contrasting ideas, but are followed by noun phrases.

Whereas is not separated by commas.

The first house looks nice whereas the second one looks horrible.

I don’t have a lot of money. However, I will go to the cinema with you.

Although it rained a lot, they enjoyed their holiday.

Expressing reason and cause

Because, as, seeing that, since ⇒ introduce a sentence. For subordinate sentences, because goes to the final position.

Because of, on account of, owing to, due to ⇒ introduce a noun phrase.

Since it’s still raining, we will cancel the match.

Because of the rain, we will cancel the match.

Expressing purpose:

In order to, so as to + infinitive

In order to that, so as to that + sentence

I am writing to you in order to complain about your hotel.

I am writing to you so that I’d like to complain about your hotel.

Exemplification

For example, for instance. Such as ⇒ refers to the last idea that was said.

We can buy some tropical fruit, for example kiwi, mango, coconut…

Most of the things in my bedroom were broken such as the taps, curtains…

Expressing consequence 

Consequently, as a consequence, as a result, therefore,

As a consequence of, as a result of ⇒ are followed by a noun phrase.

So ⇒ introduces a sentence without commas.

Consequently, I would like to receive a refund.

As a result of your poor attention, we are making a complaint.

I am complaining so I will receive a refund.