For any sentence, if the pronoun optional? Like for all verbs can we include, and/or exclude the pronoun if we choose?
Perhaps yes! But if the sentence is rejected then you will need to use the report tool to add the sentence. ;)
I really don't know and I am freaking out. I usually don't use it because I believe that the verb contains the subject (STROVE method). When I started the course I used to write all the pronouns and of course, my answer was incorrect. It would be nice if someone could give us a hint :)
The pronoun is optional in most sentences because the verb structure already contains the 'pronoun' morpheme.
Most of the time, the pronoun is optional and just used for emphasis. There are a huge number of languages like this. I see that you're at level ten in Portuguese and Portuguese has this feature too. You can say Falamos português (Tunasema kireno) without the nos but if you're emphasising or contrasting, you'll probably use it: Eles falam inglês mais nos falamos português. (Wao wanasema kiingereza lakini sisi tunasema kireno.)
(Hopefully I haven't made mistakes in either language there.)
There are times in Swahili where the verb doesn't have a subject prefix though, such as with the copula ni or the habitual aspect prefix hu-, which I guess will come up much later in this course, and in those cases, the pronoun is the only way you can make it clear who the subject is.
I'm no expert, but from what I've read, Brazilian Portuguese (which is what Duo teaches) is moving away from pro-drop, at least in the third person. It almost sounds like it's intentional to avoid ambiguity.
Spanish is definitely pro-drop, and Eric is level 8.