@whenturtlesfly : "Generally verbs of sensation don't take the present continuous, except in some constructions." (http://english.stackexchange.com/questions/5904/in-what-case-you-would-say-i-am-seeing-instead-of-i-see)
@Levi: Thanks for the reply! But yes, those are the instances that I was thinking of, so I still don't understand. You can say to a patient, "I am seeing you (on) Monday" and that is correct. In that case, "I will see you Monday" has a slightly different meaning (almost dismissive, less of a command). So, if we are relating English back to German, I am still confused as to why it is incorrect, especially as "sehe" means "am seeing" or "sees."
It isn't incorrect because there are exceptions and this is one of those situations where, depending on context, either "I will be seeing" or "I am seeing" could be correct. For example: "When are you seeing the doctor?" could be answered by either "I am seeing the doctor on Monday." or "I will see the doctor on Monday," There is a slight difference in meaning. "I am seeing the doctor on Monday." suggests a time has been arranged to see the doctor while "I will see the doctor on Monday." is more that I will arrange to see the doctor. I have reported it because it should be an alternative translation..
Perhaps one of the frustrations with language learning is that there is not often a 1-to-1 translation of a given phrase. If I am talking to "euch" (that is, more than one casual "you", so likely friends), I wouldn't say in English, "I am seeing you all/guys (on) Monday." I would say something like "I will see you Monday," or even simply, "See you Monday."
As Levi stated above, the present progressive is not something often used in German. In English we tend to say a lot of "am seeing", "is doing", "are going". So something like "Ich gehe nach Hause" could mean: I am going home, I do go home, I go home, I will go home.
@jess1camar1e: Thanks for the German explanation! I am just confused because I think my English translation should be accepted. I think there are contexts in English where you say "I am seeing you guys on Monday." Such as, "We won't see each other for a long time after this." To which you reply: "Don't you remember? I am seeing you (guys) on Monday." If you use "I will see you Monday" or "See you Monday" it is slightly different.
I can see your confusion in that case, and you would say the same thing in German (Errinert euch nicht? Ich sehe euch Montag.). However, the English in that case could also be "Don't you remember? I'll see you (guys) Monday." Hopefully you submitted your translation by reporting a "problem" with this phrase.
@whenturtlesfly : I can't give another explanation as the above rule is something I've learned not long ago (English is not my first language). Also, I would like to add another thing I've read in the comments section: Germans use the present tense quite a lot to express future actions.
Yes. "Ich trinke Milch" can mean "I drink/am drinking milk," or it can imply a future event: "I will drink milk."
So with a phrase or sentence like "Ich sehe euch...", I could say that to kids I've just found in a game of hide-and-seek (present tense) or I could say it by itself casually to friends I know I'll see again at any undefined point (I'll be seeing you), or I can add a time frame or location to imply a future event (Montag, nächste Woche, im Kaffeehaus, bald, später, etc.).
But when using the present tense to refer to future events, is it necessary to include an element of time? E.g. "Ich sehe euch" = I see you (Present). "Ich sehe euch morgen, ...heute Abend, ...Sonntag, etc" = I will see you... (Future). Can I just say "Ich sehe euch." to refer to the future?
http://www.dartmouth.edu/~german/Grammatik/Nouns/accusative.html (Uses of the accusative case, 4)
German often uses the present tense to imply future action. This is especially clear when there is a future time frame (like an upcoming day, next year, etc.). Here the upcoming Monday is mentioned, which tells us this is definitely a future occurrence. In English we would say 'I will see you Monday' because we use 'will' to signify a future occurrence. In German, we can use the present form to imply the future without a problem.