One thing I tell students in every lesson: do not learn English by translating word for word from Vietnamese.
It sounds obvious, yet this is precisely why so many people study for years and still feel they do not sound natural. English differs from Vietnamese not only in vocabulary and grammar, but in how an event is perceived and described.
A familiar example
Plenty of people reason that "tôi đang thấy" must be I am seeing. Native speakers do not say that.
With perception verbs like see, hear and smell, they say I can see, I can hear, I can smell. In the past, I saw, I heard.
And when they want the ongoing act of directing their eyes at something, they use look.
I am looking at the page, and I can see two questions.
One sentence, and the distinction is plain: look is the deliberate act of pointing your eyes somewhere, while see is the result, the image registering.
Why I teach it this way
I try to explain English the way native speakers actually conceive of it, rather than handing over a rule to memorise.
Once you grasp how the language organises things, you stop wondering whether a particular verb takes the continuous. You simply know why they say it that way.
Understanding the mechanism holds far longer than trying to remember a list of formulas.