I'm very confused about tenses.I don't know which one to use!For example what are the differences bewtween these two;

1-It is raining since 3 o'clock
2-It has been raining since 3 o'clock

and when do we use "appreciate"?
Number 2 is correct. The Present Perfect Progressive (that's the tense used in the sentence) is used to refer to an action that startet at some point in the past and which still continues at the time of speaking (at the time that sentence, in this case, was uttered).
The sentence means "It started to rain at 3 and it is still raining".
And the difference between both sentences is that No. 1 is incorrect in English.

"appreciate" has several meanings (somewhat similar to one another).
according to the dictionary, it means:

1. To recognise the qualities or worth of.
"She appreciates good wine."
"His abilities were not appreciated in his job."

2. to understand fully.
"I appreciate that this is not an easy decision for you to make."

3. to express thankfulness or gratefulness.
"I appreciate your help."
"I'd appreciate it if you would turn the radio down."

In the above cases, "appreciate" is a transitive verb. It has one more meaning, and this is as an intransitive verb:

4. (with IN) To increase in value over a period of time (property, possessions, etc.)
"Houses in this area have all appreciated (in value) since the new road was built."
The opposite here is "depreciate" (to decrease in value).

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