Translation:My uncle advised that I inform him about my sister.
Why is inform in the imperative here? The notes say:
The -u ending is used when ordering / inviting someone else to do something -- or when telling or suggesting to ourselves what to do!
In this case my uncle is advising me, not ordering, requiring nor inviting me. Shouldn't we be using the infinitive here?
Are the notes wrong, or incomplete, or is the sentence wrong?
There is one other possibility, and that is that we are using doublespeak, where to advise implies a threat. If this were the case, then I certainly wouldn't want an uncle like that!
It's one of the functions of the -u form but not the only one. If you look at Tips Notes more closely, you can read:
THE -U ENDING IN SUBORDINATE PHRASES
We also use the -u ending in subordinate phrases (clauses) starting with ke, when the verb in the preceding, main part of the sentence expresses a want, desire, demand or preference:
Isn't it preference? Anyway, it says what something should be/what should be done not what it is.
Citing form Wikipedia:
The sentence containing the deontic modal generally indicates some action that would change the world so that it becomes closer to the standard or ideal.
The only advice worth having is that which is given disinterestedly. I have certainly given advice suggesting that the advisee do something that I would prefer that he didn't do. So, no, it is not a preference. It might well be deontic modality. (Thanks for that.)
Anyway, I'll just have to get used to it. Advice is a suggestion and the notes specifically say "... suggesting to ourselves what to do ...". I should, therefore, have worked out that a suggestion to other people also takes the imperative.
If you use "informed" (indicitive past-tense), it would mean that I ALREADY informed him in the past.
If you use "inform" (imperitive), it would mean that the uncle is advising that I SHOULD inform him.
Are you trying to say that it should happen, or that it already happened?
No, because the English word "informed" (as you suggested) is "indicitive past-tense", meaning it indicates that something happened in the past.
Instead the sentence is using "inform" in the imperitive case. Imperitive case shows that something is being demanded or saying that something should occur.
In Esperanto the imperitive case is shown by the "-u" in the Esperanto word "informu".
In English, I believe that the imperitive is determined by context (it is less obvious compared to Esperanto).