activities vs. activity?

The word “activity” can be either plural or singular but the meaning is different. Pls help me distinguish the difference, when should we use it plural and when singular?
By the way, is this sentence correct?
The company’s main activity is producing and selling mobile phones.
I mean the subject and verb agreement.

The sentence is correct.

“Activity” can refer to the condition of being active or a single instance of activity.

Physical activity is good for your health. - Condition
There was much activity downtown last night. - Condition

Instances of activity may be singular or plural, and as with anything else, this depends on how many you’re referring to.

I have three activities planned for the children today.
I have one activity planned for the children today.

Mordant, thanks for your promt explanation.
I think I get your point but I’m wondering if the sentence is referring to two instances of activity, one is producing and the other is selling mobile phones.

It depends on the view of the speaker, and in this sentence, it is clear the speaker sees those acts as one. Use of the plural would also make sense.

Thank you Mordant :slight_smile: