Learn English – When can Past Continuous become the Past Perfect Continuous in reported speech

If the situation reported is still valid at the time of reporting, backshifting is optional. See Swan (2005) Practical English Usage, pages 275-6, for several examples.

John said that he would give me a pen. Correct in all situations.

John said that he will give me a pen. Possible if John has not yet given the pen to the speaker.

S2: I wanted to let you know that he had sent you an email. Correct in all situations.

S1: I wanted to let you know that he has sent you an email. Possible if the sending was so recent that 'you' may not yet be aware of this.

According to the British Council:

It isn’t always necessary to change the tense. If something is still true now ... we can use the present simple in the reported sentence.

Note the word necessary. In many contexts, it is still permissible to backshift the tense, even if something is still true now.

She said, "When do the banks close?"
1) She asked me when the banks closed.
2) She asked me when the banks close.

The first version (with a backshift) is definitely the best choice if we are talking about a conversation that took place many years ago, as bank closing times may have changed since then. The second version is better if you are talking about a conversation that took place last week, but you could also use the first version.

He said to me, "What does this word mean?"
1) He asked me what this word meant
2) He asked me what this word means

It is very unlikely that the word has changed its meaning since the conversation took place, so either version is acceptable. I think that I would go for the first version (with a backshift), though I cannot explain my preference in any way.

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Note that, unless you want to specify when you learnt about something "I have known about it for some time", you generally use know in the present tense: "I know that..."