- She always sees things that don't exist.
- She is always seeing things that don't exist.
In the first sentence we use verb "to see" in the present simple tense. In the second sentence, verb "to see" in the present progressive, unusual for verbs that express forms of sense perception. Are both sentences correct and interchangeable? Any subtle difference in meaning between them?
Best Answer
The second sentence expresses the speaker's opinion, it suggests an annoyance or irritability at the frequency with which the woman is seeing non-existent things. The present progressive tense is used when we mention the activity, and with many senses we can use this structure.
Now is the present simple tense and the progressive tense always interchangeable? No, it isn't. Compare:
A: What do you do?
B: I taste wine for a living.
A: What are you looking for?
B: My glasses. I can't see a thing without them.
A: If I told you once, I told you a hundred million times. Don't touch the DVDs with your grubby fingers.
B: Sorry...
A: They say dogs can smell cancer.
B: Really? I never knew.
A: Can you hear it? There's a noise downstairs.
B: It's nothing. Now go back to sleep.
John Lawler in an answer posted a succinct explanation on volitional and non-volitional verbs
Hearing: You listen to something on purpose, but you can hear it by accident.
Vision: You look at something on purpose, but you can see it by accident.
Verbs for the other three senses don't vary; you can smell, taste, or touch/feel on purpose or not.
Therefore with non-volitional senses such as sight and hearing, we often use can and the verb remains in the present simple, the progressive tense is more common with things we do on purpose.
Without going into greater detail, I find that Wikipedia explains well some of the differences between the present simple and continuous