I know you could just say because the $\det = 0$.
But during the introduction of determinants the professor said, obviously if two columns of the matrix are linearly dependent the matrix can't be inverted, therefore it is zero. He made it sound like it is an intuitive thing, a simple observation, but I always have to resort to the properties of determinants to show it.
How does one trivially see that you can not invert a matrix without a full rank?
Best Answer
Suppose that the columns of $M$ are $v_1, \ldots, v_n$, and that they're linearly dependent. Then there are constants $c_1, \ldots, c_n$, not all $0$, with $$ c_1 v_1 + \ldots + c_n v_n = 0. $$ If you form a vector $w$ with entries $c_1, \ldots, c_n$, then (1) $w$ is nonzero, and (2) it'll turn out that $$ Mw = c_1 v_1 + \ldots + c_n v_n = 0. (*) $$ (You should write out an example to see why this first equality is true). Now we also know that $$ M0 = 0. (**) $$ So if $M^{-1}$ existed, we could say two things: $$ 0 = M^{-1}0 \ (**)\\ w = M^{-1} 0\ (*) $$ But since $w \ne 0$, these two are clearly incompatible. So $M^{-1}$ cannot exist.
Intuitively: a nontrivial linear combination of the columns is a nonzero vector that's sent to $0$, making the map noninvertible.
But when you really get right down to it: proving this, and things like it, help you develop your understanding, so that statements like this become intuitive. Think about something like "the set of integers that have integer square roots". I say that it's intuitively obvious that $19283173$ is not one of these.
Why is that "obvious"? Because I've squared a lot of numbers, and all the squares have a last digit that's either $0, 1, 4, 5, 6,$ or $9$ (because those are the last digits of squares of single-digit numbers). Now that I've told you that, my statement about "intuitively obvious" is obvious to you, too. But until you'd at least learned a little about integer squares by investigating them, your intuition wasn't as good as mine. Sometimes "intuition" is just another name for "applied experience."