Translation:El verano pasado
it is frustrating that often adding the word "the" is counted as incorrect yet her there is no the, just last summer and if "the" is not included it is incorrect. There does not appear to be a pattern.
It's just a grammatical difference between languages. Unlike English, in Spanish you need a definite article for seasons. The exception is if the season follows "de" "en" or a form of "ser". See below for some general rules regarding the use of definite articles in Spanish:
The other dilemma here is that, while the definite article is needed in a full sentence or in a response to a question, this is a sentence fragment (no full stop), so it's a guessing game as to how much of the fragment Duolingo wants.
It's just a part of the language. Seasons and months have the article regardless of whether or not it's included in the translation
They usually don't allow spelling mistakes if it results in another Spanish word. In this case, "pesado" means "heavy."