Questions & Answers about Улица короткая, а не длинная.
Why is а used here instead of и (and) or но (but)?
The conjunction а is used to show a direct contrast, often translating to "but rather" or a contrasting "and". Here, it pairs "short" with its opposite, "not long". You wouldn't use и because that is for adding similar or cooperative things together. You usually wouldn't use но here either, because но introduces a conflict or a surprising exception (like "The street is short, but we got lost").
Where is the word for "is" in this sentence?
In the present tense, Russian almost always drops the verb "to be" (am, is, are). To say "The street is short", you simply put the subject and the describing word together: "Street short" (Улица короткая).
Why do the words короткая and длинная both end in -ая?
These are adjectives describing the noun улица (street). Since улица is a feminine noun (which we can tell because it ends in -а), the adjectives that describe it must also be in their feminine form. For most standard adjectives, the feminine ending is -ая.
Is the comma before а required?
Yes! In Russian punctuation, you must always place a comma before the contrasting conjunctions а and но.