"Groß" means big. In this case, the noun it is describing is in the nominative case, and there is no article (i.e. no the/a) so, the adjective ending for a masculine noun in the nominate case is "–er". Therefore, it is "großer" — "ihr großer Rock ist rot" (her big skirt is red). "Großer" never means bigger, it is simply "big" with the correct adjective ending for this case. If we were doing it with a feminine noun, i.e. "her big newspaper is red", we would say "ihre große Zeitung ist rot" since "–e" is the adjective ending for feminine nouns (regardless of the article, actually!) in the nominative case.
"Größer" (with the umlaut) means bigger. If you were to use it in this case, since the noun it would be describing is in the nominative case, and as we said, there is no article, once again the adjective ending for a masculine noun in the nominative case is "–er". Therefore it would be "größerer" — "ihr größerer Rock ist rot" (her bigger skirt is red).
Does this make it clearer? Duolingo doesn't do a great job of explaining adjective endings and the cases. It might make sense to look elsewhere for an explanation. I think learning the tables for adjective endings with the definite article, indefinite article and no article is a good idea (they are all slightly different).
This site always helps me. http://www.deutschseite.de/grammatik/adjektivendungen/adjektivendungen.html
That would be Ihr größerer Rock ist rot.
In this sentence Ihr (the determiner) is weak so the adjective must be strong. The strong endings are the ones der and dieser use. The weak endings are just -e and -en.
This is easier to explain with charts: http://gregreflects.blogspot.com/2015/02/how-to-memorize-german-cases.html
No; ihr großer Rock is in the nominative case, as the subject of the verb.
ihr has no ending for gender, number, or case, so the following adjective has to show the gender, number, and case, thus taking the -er ending for masculine nominative singular.
(After possessive determiners such as ihr or mein, adjectives take mixed inflection like after ein or kein.)
ihr is the possessive determiner corresponding to sie, so it can mean both "her" and "their", just as sie can mean both "she" and "they".
And Ihr (capitalised) is the possessive determiner corresponding to Sie, and so it means "your".
Remember that du is used when speaking to one person whom you know well, ihr when speaking to several people whom you know well, and Sie when speaking formally/politely to one or more people.
It is a bit more complicated than that:
- possessive pronoun (ihr) is preceding the adjective = mixed inflection
- "der Rock" is a masculine noun, singular form
- nominative case (it answers the question "Was ist rot? = What is red?")
mixed inflection + masculine gender +singular + nominative case = the ending for the adjective is "-er"
No, it is not nominatiive, not in German neither in English. "You skirt" is not correct in English either.
But partly you are right - "Ihr Rock" with the "I" capitalized in the first word of the sentence can mean either "her skirt" or "their skirt" or "your (formal Your) skirt".