Can this mean that we cannot take his place, or mean that we cannot find a replacement for him, like the English sentence can? Or would one of these meanings be expressed differently in German?
Could I say "Wir können nicht ihn ersetzen"? I still don't understand what the position of "nicht" should be.
Based on the link from @allintolearning, the explicit answer perhaps can be the following.
We can say "Wir können nicht ihn ersetzen" ("ihn" is negated), but that would translate as "We can replace not him (but possibly someone else)", or "As for him, he cannot be replaced by us".
On the other hand, the original sentence ("Wir können ihn nicht ersetzen") says: "As for replacing of him, we cannot do it".
The third option is "Wir können ihn ersetzen nicht" (the whole sentence is negated), which states: "As for whether we can replace him, we cannot".
I think that only the second and the third option are equivalent.
Ich denke, dass du meinst "wir können ihn ersetzen nicht" für deine dritte Möglichkeit. So wie es jetzt ist, die zweite und die dritte sind identisch.
Regarding *Wir können ihn ersetzen nicht, I can see now. A relevant quote from the linked page (http://www.dartmouth.edu/~deutsch/Grammatik/WordOrder/MainClauses.html):
« If, on the other hand, we wish to negate the whole general idea of the sentence, we put the nicht after modifier, at the end of the sentence: Wir fahren am Montag nicht. [Some more examples are given.]
If the sentence has a verb complement (verbal idea), however, that will be the part that is negated:
Er spielt nicht Schach. = Er doesn't play chess.
Mein Großvater fährt nicht Auto. = My grandfather doesn't drive.
Consider this last example: Mein Großvater fährt nicht Auto. [...] the concept here is Auto fahren. Auto, in other words, is the verb complement, necessary to the predicate's meaning, and so it goes to the end of the sentence, with the nicht preceding it.
Were the auto conceptually the object of driving, i.e. an augmentation, rather than a necessary part of the predicate, then the sentence would read: Mein Großvater fährt dieses Auto nicht. »
So I guess the logic of nicht in German is the following. Nicht can be placed at the very end, before the point (negating the whole point of the sentence), but only when it is unavoidable.
In Mein Großvater fährt dieses Auto, the main idea (the predicate) is fährt, but the sentence can't be * Mein Großvater nicht fährt dieses Auto because the main verb should stay on the second place. So putting nicht at the end is unavoidable indeed.
On the contrary, the main idea in Wir können ihn ersetzen is können ersetzen (it can't be narrowed down to just können), so it's enough to negate a part of this (anyway conceptually individable) main idea: können nicht ersetzen (the surface form) - nicht können ersetzen (the meaning) - not able to replace (instead of the apparent able to not replace). The idea of able to not replace would be expressed by some other means.
Finally, Ersetzen können wir ihn nicht is possible (right?) since nicht can't be put before ersetzen or können without shifting the main verb können from its second place.
Does that make sense to a German speaker?
When do you say can and when do you say could? I always thought those were the same thing..
"Could" is past or subjunctive of "can."
For example: "Yesterday, I could walk [past tense], but today I broke my leg, and now I can't walk [present tense]."
Or: "If I had crutches, I could walk [subjunctive], but I don't have crutches, so I can't walk [indicative]."