For native speakers of some languages, the most challenging aspect of English is how to use commas correctly. Here, I will tell you about the most specific cases.

1. Never use a comma before THAT. It does not matter what type of THAT it is. Just remember not to use a comma before any THAT. * I do not mean some connectors where 'that' is a part, like 'that is why', etc.

e.g.: These measures have been implemented so that the crime rate could go down.

I know the author that has written this wonderful book.

2. Never use a comma in the middle of a complex sentence before subordinate clauses of time/condition/cause/etc. (it means that you should not use a comma before BECAUSE, AS, SINCE, IF, UNLESS, AS SOON AS, ONCE, WHILE, BEFORE, AFTER, etc.)

e.g.: It was permitted because it improved the results.

I will tell him about it when he comes home.

Please contact me if you have any questions.

*However, if you start your sentence with a subordinate clause of TIME, CONDITION or REASON/CAUSE (which means that you will start it with the conjuctions mentioned above), use a comma after it and before the main clause. To make it easier, let us just reverse the examples above:

e.g. Because it improved the results, it was permitted.

When he comes home, I will tell him about it.

If you have any questions, please contact me.

3. When you use ADVERBIALS of manner, time, place, addition and contrast in the beginning of a sentence, separate them with a comma.

e.g. Yesterday, they presented the plan.

All of a sudden, somebody opened the door.

Moreover, her first attempt failed.

Quickly, they finalised their project.

Nevertheless, the measures were taken last year in order to prevent the spread of the disease.

(to be continued)