Learn Russian: Эта духовка очень удобная.

In Russian, the present tense of the verb "to be" is almost always left out. So, Эта духовка (This oven) очень удобная (very convenient) completely stands on its own to mean "This oven is very convenient."

Why do the words Эта and удобная have those specific endings?

They are matching the gender of the noun they describe. духовка (oven) is a feminine noun because it ends in -а. Therefore, the word for "this" becomes the feminine Эта and the adjective takes the feminine ending to become удобная.

I previously learned that удобный means "comfortable." Why is it translated as "convenient" here?

The adjective удобный actually covers both meanings. When talking about furniture, shoes, or clothes, it usually translates to "comfortable." But when talking about tools, appliances (like this oven), locations, or schedules, it means "convenient," "handy," or "easy to use."

How would I change this sentence to say "This oven WAS very convenient"?

Because you are talking about the past, you can no longer skip the verb "to be." Since духовка is feminine, you would insert the feminine past tense verb была, making the sentence: Эта духовка была очень удобная.