Yes, you can use "open the curtains".
If your two examples are sentences, then yes, they should be capitalized.
Student or Learner
We can use "close the curtains".
Can we use "open the curtains"?
Should my sentence in the inverted commas start with a capital?
I would add the word 'please' to either end of your sentences.
"Please open the curtains." "Close the curtains please."