Ĉu

In the Wellington Esperanto group the other day, I got pulled up on my use of ĉu; I’m sure it’s not an uncommon problem, but I shall put Cox’s notes here for reference.

Ĉu = Whether. An interrogative adverb used at the beginning of a direct question; it has no English equivalent. Ĉu simply shows that the sentence is interrogative, and therefore, to translate it, we must look to the tense of the verb and to the subject, so as to preface the interrogation with “do,” “does,” “did,” “have,” “has,” “is,” “will,” “shall,” “was,” “were,” “should,” “would,” “can,” etc. etc. Examples.—Ĉu vi komprenis? = Did you understand? Ĉu Johano skribas? = Is John writing? Ĉu vi povas fari tion? = Can you do that? (see remarks on Interrogation, par. 58). Ĉu = whether (or if in the sense of whether) (a conjunction) is used in indirect questions, when we generally use “if.” Examples.—Diru al mi, ĉu li venos = Tell me if (whether) he will come. Diru al mi, se li venos = Tell me if he comes.
Ĉu … Ĉu = Whether … whether. Example.—Ĉu li venos, ĉu li ne venos, Paŭlo foriros = Whether he comes (will come) (or) whether he does not (will not come), Paul will depart.

Cox, George. The International Auxiliary Language Esperanto Grammar and Commentary

Ĉu = Whether, or, if (see remarks on ĉu in the list of adverbs, par. 248 (b)).
Ĉu … aŭ = Whether … or. Example.—Mi iros, ĉu li venos aŭ ne = I shall go, whether he come (will come) or not. Ĉu … ĉu = Whether … whether. Example.—Ĉu li skribos, ĉu li ne skribos, mi ne respondos al li = Whether he writes, whether he does not write, I shall not reply to him.

Se = If. Example.—Mi ĝin faros, se mi povos = I shall do it if I can (shall be able). Se ne = If not, otherwise, lest. Example.—Lernu vian lecionon, se ne, mi vin vergos! = Learn your lesson, otherwise I shall cane you! Se nur = If only, provided that. Example.—Vi vidos Johanon, se nur vi alvenos frue = You will see John, provided that you (will) arrive early. Se okaze = If perchance, in case, in the event of. Example.—Se okaze vi venus malfrue, Johano ne estus tie ĉi = If perchance you should come late, John would not be here. Se tamen = If however, if still, if though, if notwithstanding. Example.—Se tamen vi ne povos veni morgaŭ matene, venu vespere = If, however, you are not (will not be) able to come to-morrow morning, come in the evening.

Cox, George. The International Auxiliary Language Esperanto Grammar and Commentary