I have been wondering if one could use the Present Progressive to express what is really going on when something happens. For example: "When you present something to the class, you are expressing your ideas, which is why public speaking is good". Do I need to say: "You express your ideas"?
As with many choices of aspect (eg continuous or not; perfect or not), both are possible, and the difference is not in any objective circumstance, but in how the speaker wishes to present the temporal relationships and focus.
If the speaker says you express, this is a plain statement of what happens, with no special focus.
If the speaker says you are expressing they are are focussing on the activity of expressing ideas: in a way, they are inviting the hearer to mentally be present with the person presenting, and travel that "journey" with them. It invites a closer emotional involvement with what is being described.
You express your ideas.
You put forward an argument why public speaking is beneficial for an individual. It's an idea, your opinion. You don't describe any situation in particular.
You are expressing your ideas.
It creates the impression for the listener that the event is happening now. It makes the idea more real and helps to hold people's attention.