I've recently corrected a text that one of my students wrote. One of his sentences was, "Sometimes it's annoying that I don't learn new things as fast as I want..." I replaced as fast as I want with as fast as I'd want to/ as fast as I'd like to. However, he asked me for an explanation and I am stuck. Am I wrong? Are both constructions correct? If not, does anyone have an explanation as to why the first is incorrect?
Both your student’s phrase and yours are valid (which means you were wrong to correct his). There are numerous other variants—you could include “would” (the ’d) or not, you could include “to” or not, and you could use “like” or “want” pretty much interchangeably. All of them work out grammatically and with about the same meaning.
I don’t know how to meaningfully explain why these are all correct—unlike something that’s incorrect, where I could point to a flaw, here I’m just left looking at both and thinking “there’s nothing wrong with any of these.”
"As fast as I want (to)" means "As far as I want to ''at this moment''".
"As fast as I'd want to" (or "I'd like to") tends to bear the added implication that this is "as fast as I would ''ever'' want to."
In more sensitive contexts, the second option may also bear a note of moral judgment: "The speed limit is as fast as I'd want to drive" suggests that one may have moral scruples against breaking the law.
However, if one is a habitual speeder, but today the weather conditions are particularly foul, your passenger may wonder at your reluctance to go faster.
"The speed limit is as fast as I want to drive, given the fact there are sheets of water on the road and my wipers are going as fast as they can."