In the textbook, I saw both "现在几点？" and "现在几点了？"
I know they are both correct and "了" often indicates past tense, but what's the difference here?
I think it would be really good if I can look up the usage of "了". But I didn't find any good online dictionary.
了 http://www.cantonese.sheik.co.uk/dictionary/characters/460/
[2] [final particle] indicating change of situation
"现在是下午五點" = "It is 5:00 pm now."
"现在是下午五點(了)" = "It is 5:00pm now" (final particle 了 indicates 'the current time has reached 5:00pm'. Situation changed from 'not yet 5:00pm' to 'is 5:00pm' )
Similarly:
"现在几点？" = "What time is it now?"
"现在几点了？" = "What time is it now?" (final particle 了 indicates 'what time the current time has reached'. Situation changed from 'not yet the current time' to 'is the current time' )
The final particle 了 is mostly used to soften the tone of a sentence to make it less blunt. Also it formally indicates the sentence is 'completed' (that's why it is called 'final particle').
As a native speaker, I really can't tell the difference. You might add "了" to make you sound a little less rude, but I really can't feel any difference.
As others have already said, adding 了 indicates a change of state. Here is an example of when you might want to add 了, and when its not necessary:
If you don't have a watch and are wanting to ask a friend what time it is, you can simply ask 现在几点？
Now imagine you are waiting for midnight on New Years eve. You ask your friend what time is it? 现在几点？Then 10 seconds later you ask again, but this time you ask 现在几点了? You can add 了 this time, as we know the time has changed but still don't know exactly what time it is. You can imagine it in English, "What time is it?" (10 seconds later) "What time is it now?" or "what about now?"
了 does not indicate past tense, but perfect tense for action verbs.
A similar question was asked here: https://chinese.stackexchange.com/a/18075/13501
Grammatically, in your case, 了 indicates the change of state (English equivalent: become ...)
However, in daily use it only softens the tone of the sentence.
And an older post here fyi: https://chinese.stackexchange.com/a/18061/13501
对于我来说，没有区别，加“了”更适合日常习惯，没有也可以。 我是一个普通的中国人。
