Tarde introducción...

[Tomisimo]

Welcome I'm not Rusty, but I'll give you a few answers anyway.

Originally Posted by laepelba

I have done some reading on "hace ... [tiempo] ... que", including a link that I found that you posted in a different thread, and I think that I'm good with that construction. One quick question, though. If the verb that follows "que" is present tense, then it indicates something that has been ongoing? Like "Hace unas semanas que uso este foro..." where "uso" is present tense, so my use of the forum has been ongoing for several weeks. Right? But later in that same first paragraph, it says "Hace veinte años que estudié español...", the verb estudié is past tense, therefore this is something that happened twenty years ago and is NOT ongoing. Right?

Yes, by using the present tense, you are indicating that the action is ongoing. In your second example, when using the preterit, the "que" is optional.

Hace veinte años que estudio español = I have been studying Spanish for 20 years.
Hace veinte años (que) estudié español = I studied Spanish 20 years ago.

Originally Posted by laepelba

In the first sentence, I wrote "pero no me presenté todavía" and you changed it to "pero todavía no me he presentado". I have two questions about that construction. First - is the "todavía" always at the beginning of the phrase where it's used? Second - what does the "he" signify? Is it "here"? Or is it a conjugated verb that goes with the "presentar" (does it mean something like "have")? If so, what is the infinitive of that verb? And is the infinitive of "presentado" actually "presentarse"? AND ... is it similar in use to what you said a couple sentences later when you change what I wrote to "pero se me ha olvidado todo".

"todavía" can be used either before or after the verb, but using it before the verb might be more common. "he" is the first person singular conjugation of "haber"; "presentado" is the past participle of "presentar":

No me presenté = I didn't introduce myself.
No me he presentado = I haven't introduced myself.

Originally Posted by laepelba

Finally (for now), in that last sentence to which I just referred ("pero se me ha ovidado todo"), why is there both a "se" and a "me"? Is that a unique thing with "ovidar"?

In Spanish it's more common to use a passive construction (which looks just like a reflexive construction) for some verbs, for example olvidar. It's not very common to say in Spanish "I forgot it". You would basically say "It was fogotten on me". In other words, it wasn't your fault, or you didn't forget intentionally. If you say "I forgot it" (Lo olvidé), you're implying a willful forgetfullness. Instead of doing this, you use this passive construction:

Se me olvidó = I forgot it = (literally) It was fogotten on me.

"se olvidó" is the "it was forgotten" part, and you insert "me" meaning that you were the affected party.

"se me ha olvidado" is exactly the same, but it's using the perfect tense:

Se me olvidó = I forgot it
Se me ha olvidado = I have forgotten it

Hope that helps a bit. (Or maybe Rusty should answer )