Hi Trang, you may wish to develop your sentences a little to define 'when' you are talking about.
For example
I prefer tea to coffee ( a statement of fact, always true)
I would prefer a tea, rather than a coffee. (present tense, someone has just asked you what you would like to drink)
I used to prefer coffee, but now I only like to drink tea (past events relating to the present situation)
Keep up the good work!