You can spend your life writing or speaking short sentences, but (and
that's a conjunction) if you want your English to flow a bit more
naturally you need to learn how to join sentences together and (another
conjunction) you need some linking words to do this.
Basically a conjunction connects two words, sentences or clauses together:
although, and, because, but, if, or, so, unless, when, while ...
There are two types of conjunction: coordinating conjunctions and subordinating conjunctions.
You can also use conjunctions in pairs, these are called correlatives.