"It is my light dress."
Translation:C'est ma robe légère.
The adjective places after the noun is the rule for 90% of cases. However, as TominoCZ says, you have a few exceptions when the meaning of the adjective is related to Beauty, Age, Good and bad, Size (except for grand with people).
"légère" is not part of the BAGS, so it follows the basic rule. - une belle robe - une vieille robe - une bonne robe - une petite robe ---> une robe légère
Those words are possessive adjective, defining who the owner is.
To each person of conjugation, there is a specific possessive adjective which has to agree with the object owned, in gender and number:
- je -> mon chien (masculine singular), ma chienne (feminine singular), mes chiens (plural common to masculine and feminine)
- tu -> ton, ta, tes
- il/elle/on -> son, sa ses
- nous -> notre, nos
- vous -> votre, vos
- ils/elles -> leur, leurs
This is a rule you will have to apply often on Duolingo.
In French, "c'est" (sing.) and "ce sont" (plural) are used in a large variety of expressions, when a pronoun (it, she, he, they) is subject of verb "être" and followed by a nominal group, ie: article (+ adjective) + noun.
it is + noun => c'est + article + noun
she is + noun => c'est + article + noun
he is + noun => c'est + article + noun
they are + noun => ce sont + article + noun
By the way, "it" woud not translate in "il" since "une robe" is feminine : ma robe est rouge, elle est rouge.
Although asked several times here, we did not receive an answer as to why "...my robe claire" is not accepted. Please explain why this is not a correct answer. It doesn't matter how the English say light colored dress. We want to know how the French say light dress, as in light-colored. Thank you.