I suppose I need to clarify a bit, and that's completely my fault because of how emphatically I made my original statement. There's definitely a give and take between me and the players and while I'd obviously prefer not to have to explain "this is how you make an attack" to someone who's been playing their fighter for five levels, I just don't get too worked up about it either if it does happen.
I sort of look at myself as a bit of a "teaching" DM. My innate interest in the rules and readiness to absorb them all like a sponge also makes me keen to share that knowledge without having my players go digging through the rules on their own. If they want to that's great, and I will encourage it even, but I don't necessarily expect it and I don't get myself bent out of shape if it doesn't happen.
It's a bit difficult to explain I guess, with new players or even vets that might just be new to a particular edition, I start out by working very closely with them not just during character creation, but also through the first few sessions. I even regularly run tutorial style one-offs prior to the actual campaign to go over basic mechanics with the players, even going so far as to tailor those one-offs to force the player to interact with different parts of the character sheet or the characters abilities so they at least having something to call back to when it comes up in actual play.
As far as how I'd approach the various situations that could come up from a players lack of knowledge, it's too situation-specific for me to give a better answer than: I'm here to help and will always do so, including reminding players of what their characters are capable of should they forget... as long as I can do so without over-influencing their actions. It's a hard line to walk, but I like to think I do ok.
I'll try and come up with a quick example or two, but these might be pretty piss poor. Say I've got a player playing a rogue and the character is in melee with an enemy and the player says something like "I don't want to be standing next to this orc when he starts swinging, I'm going to run and get behind the paladin." Now, the player never says they disengage, but if they haven't used their bonus action yet that turn? Well, chances are I'll say something like "Ok, you can run from the orc, but if you don't use cunning action to disengage he's getting a swing at you." I'll probably even point to the ability on their sheet. Now believe me, part of me would love to see the rogue eat an OA and learn the hard way, use the stick as it were, but it's just not my style.
To use the spell slot example, or one like it, say the player says something like "I'd really like to cast magic missle, but I'm out of 1st level spell slots." Chances are I'd point out that perhaps they still have other slots available to "up-cast" the spell. A more likely scenario, given the character sheets I have my players use, would be "I see here in my spell list it says something about more missles per higher spell slot used, what's that about?" And I'd explain it to them. For reference by the by I have my players use the printer friendly version of MPMB's character sheet, complete with generated spell lists for the characters who need them, it's excellent for knowing what a spell does or knowing exactly where to reference it if you need to.
I guess in the end I like teaching players and so for me the questions never really get old. I mean, it helps that no one in my groups exactly qualifies as special needs... so between my teacher-like approach, quality character sheets that I show them how to reference, and never getting tired of quesrions... My players seem to grasp the basics pretty well and fairly quickly. So while my original statement might have had a dash of hyperbole in it, I really am that keen to teach and patient with my players that I wouldn't get too worked up if they were asking basic questions at a point most might not.
Anyway, that's my ramble. There's no genuinely wrong way to do this, so even if people hold different expectations than I do it's very much a "to each their own" sort of thing. I'm just pretty laid back about it all... though I'm no carebear DM either, my statements in other threads about killing characters will stand for themselves on that subject.
