Learn Russian: Помидор красный.

In Russian, the verb "to be" is almost never used in the present tense. Instead of saying "The tomato is red," you simply put the subject and the adjective together: Помидор красный (literally "Tomato red").

Why does красный end in -ый?

Adjectives in Russian must match the gender of the noun they describe. Помидор is a masculine noun because it ends in a consonant. Therefore, we use the masculine adjective ending -ый.

Why does the adjective красный come after the noun?

If you just wanted to say "a red tomato," you would normally put the adjective first: красный помидор. Because the adjective comes after the noun here, it acts as the core of the sentence, meaning "The tomato IS red."

How do I know it means "The tomato" and not "A tomato"?

Russian does not have articles like "a", "an", or "the". You have to figure it out from context. Because this is a statement stating a fact about a specific object, "The tomato" is the most natural translation in English.