Questions & Answers about Твои кроссовки и ключ в сумке?
Why does the sentence use the plural Твои (your) when ключ (key) is singular?
Твои is plural to agree with the noun right next to it: кроссовки (sneakers). In Russian, when a single possessive pronoun or adjective applies to a list of items, it usually just matches the gender and number of the very first item in the list.
Where is the verb "are" in this sentence?
Russian leaves out the verb "to be" in the present tense. While English requires "Are your sneakers in the bag?", Russian simply says "Your sneakers and key in the bag?" You show it is a question by raising your pitch at the end of the sentence.
Why is the word for "bag" written as сумке instead of its dictionary form сумка?
Because it is indicating a location (where something is). When you use the preposition в (in) to talk about location, the noun that follows must be in the Prepositional case. For feminine nouns ending in -а like сумка, the ending changes to -е, making it в сумке.
Are кроссовки (sneakers) always plural in Russian?
Just like in English, words for pairs of shoes are almost always used in the plural. You will occasionally hear a singular form if someone is looking for just one missing shoe, but as a general rule, you should treat кроссовки as a plural word.