Present perfect tense versus Past perfect tense

How to use present perfect and past perfect tenses

The present perfect tense is used to describe an action that was started earleir and is still in progress or an action that has been done. Look at the following examples.
1 It has been raining since this morning. [It started to rain this morning and right now it is still raining]

2 I have lived here since 10 years ago. [I started to live here 10 years ago and I am still living here.]

3 The pupil has been absent from school since last Monday. [The pupil was absent from last Monday until now.]

4 I have taken my lunch. [I took my lunch quite some time ago and this act has been completed and I am just telling you this fact.]
[You should not state the time if you use present perfect tense. You can say 'I took my lunch at 1.00p.m. but you cannot say 'I have taken my lunch at 1.00 p.m.]

As for past perfect tense, it is used together with the past tense to indicate an action that was done earlier than the one mentioned in the past tense. To make this clear to readers, study the following sentences.
1 I had taken my breakfast when I went out. [Taking my breakfast happened first. Going out took place later.]

2 She had run for 20 kilometres when she fainted. [Running for 20 kilometers took place first and fainting happened later]

3 I had planned what to write in this post when I started to pen whatever was in my mind. [Planning what to write happened first and starting to pen took place later.]

So much for present perfect tense and past perfect tense.