Hello, Emonhenn, and welcome to the Grammar Exchange!
I will try to explain that sentence to you, but I strongly suggest that you continue studying English without caring too much about grammatical terminology.
I don't know if you've ever heard about "relative clauses," that is, clauses that modify nouns.
Instead of saying:
4. We are sorry for the inconvenience caused.
we can say:
5. We are sorry for the inconvenience (that) we may have caused you.
or, in the passive voice, that is, if you don't want "we" to appear again:
6. We are sorry for the inconvenience (that) may have been caused to you.
Sentences 4, 5 and 6 above refer to the past.
7. We are sorry for the inconvenience (that) this may cause you.
refers to the future possibility of some inconvenience affecting "you." "you" is the person who may suffer that inconvenience.
"that" is between parentheses in all of the sentences above because it is optional.