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I don't understand what time I should use in the following sentence:
I couldn't understand the words they have used.
I couldn't understand the words they had used.
or maybe
I couldn't understand the words they used.
Are those sentences correct? How would you say? Thanks in advance!
I couldn't understand the words they have used.
Look at the above statement carefully, it has made of two different times. 'I couldn't understand' is one and another is 'the words they have used'
'I could not understand' talks the simple past. However, 'the words they have used' is where you need to look at. Have + past participle is used when something happened in past and is still going on or simply has an effect in your present (current time)
Look at this one: I have lost my car keys. This simply means that you haven't found them yet or you still be standing out of your car and looking for it. "This is your present (current time) situation.
'I couldn't understand the words they have used' simply means you still don't know what they said.
If you say, 'I couldn't understand the words they had used' means you don't need to know now, it was a point in time where you needed to know but 'now' you don't.
I would rather suggest you to look at the time lines of the English grammar tenses.
I couldn't understand is referring to an action that took some time and that happened in the past (the meaning is hardly different from "I did not understand"). Therefore, simply use the simple past:
I couldn't understand the words they used.
Compare it to another example:
I couldn't understand the language the spoke.
The alternatives are incorrect:
*I couldn't understand the language they have spoken.
*I couldn't understand the language they had spoken.
If you're talking about a situation that happened in the past and is over:
I couldn't understand the words they used -or- were using.
This makes perfect sense because your action is in the past, and their action is in the past.
However, you can't combine the past form couldn't understand with the perfect form they have used because the premise of the perfect tense is the present, so there is no verbal coherence in this sentence unless the subject (I) is speaking in the present.
However, you can say:
I couldn't understand the words they had used.
because both of these actions relate to the past, as well as a point before the contextual past - so it's perfectly coherent.
I couldn't understand the words they had used.
"Had" implied that the action happened before another action, both of which occurred in the past. "Couldn't" is in the simple past, therefore "had used" is appropriate, since they had used the words before you tried to understand them.
