The aim in writing paragraphs is to stick to one topic, but it’s not enough just to lump together individual sentences within a paragraph.
Creating a cohesive message for the reader is achieved using transitional words and expressions.
Some transitions serve to convey time: after, before, concurrently, first, immediately, in the meantime, meanwhile, later, soon, subsequently, then.
Transitions tend to appear at the start of a sentence and are soon followed by a comma, to separate them from what follows.
If you reverse the two parts of the sentence, you don’t need a comma, but the meaning might be changed. There may be a change of emphasis. It might not make sense!
Transitions are mostly small words, or groups of words, but they can make a big difference to the effectiveness of your writing.