Ob - expresses uncertainty or enquires for things that are uncertain. Ich weiß nicht, ob ich am Wochenende zu dir kommen kann. Ob diese Entscheidung richtig war, wird die Zukunft zeigen. Kannst du mir sagen, ob er zuhause ist? Weißt du, ob sie gestern angerufen hat? Wenn - means "in case of", "in the event of". Komm mal vorbei, wenn du in der Nähe bist. Melde dich, wenn du Hilfe brauchst. Wenn ich zu spät komme, bekomme ich Ärger. Wenn du am Wochenende Zeit hast, sag mir Bescheid.

wenn - falls - ob "wenn" can be both "if" and "when" (only context let's you decide what would make sense) ⋄ Wenn er krank ist, dann wird er nicht kommen. (If he is ill, he won't come.) ⋄ Wenn er krank ist, dann jammert er immer. (When he is ill, he is wailing all the time.) "falls" equals to "if" only (this is a wee bit stronger than "wenn" insisting upon the conditional meaning) ⋄ Falls das stimmt, dann müssen wir unseren Plan ändern. (If that is correct, then we will have to change our plan.) "ob" equals to "whether" ⋄ Ich weiß nicht, ob ich das tun soll. (I don't know whether I should do this.) The last sentence also often appears as (I don't know if I should this.) I mean, I heard this often said by English native speakers using "if" instead of "whether"... Is this a correct alternative? Or is it wrong, as I am understanding Nanren888's comment?

German's my second language, Evan, not my first, but 'wenn' is used the way we use 'if' as in "If she wants dinner, she'll have to wait" (Wenn sie Abendessen haben will, muss sie warten") "ob" is 'whether' "I don't know whether I should go to the party" (Ich weiss nicht ob ich zur Party gehen soll)

"if" does not equal "whether". I don't know if I can come (or not). <-- not correct. . But, I presume the explanation about wenn and ob is correct. . It seems it is the confusion of "whether" with "if" that is causing the problem.