- Actions taking place at the moment of speaking (now) Level: elementary
He is playing football.
The action is going on now.
Signal words like now, at the moment are often used to emphasise that the action is taking place at the moment of speaking. Signal words are not really necessary, however, as this is already expressed by the tense itself.
- Arrangements for the near future Level: lower intermediate
I'm going to the theatre tonight.
In the example you can see that the tickets are already bought. So we are talking about an arrangement for the near future.
To make clear that the action is not going on now, we usually use signal words like tonight, tomorrow, next Friday, at noon.
- Actions taking place only for a limited period of time Level: intermediate
Jim is helping in his brother's firm this week.
Here we are talking about a time limit. Jim does not usually work in the firm, he is still at school and wants to earn some extra money during his holidays.
To make clear that there is a time limit, we usually use signal words, e.g. this week/month/year.
- Actions taking place around now (but not at the moment of speaking) Level: intermediate
I'm studying for my exams.
This action takes place around now and only for a limited period of time, but not at the moment of speaking.
We don't have to use signal words here, but we often find signal words in such sentences, e.g. now, at the moment.
- Development, changing situations Level: upper intermediate
The population of China is rising very fast.
The sentence describes a development from one situation to another.
Signal words are not that common here, only sometimes the change of situation is emphasised by using for example more and more.