Translation:We eat a lot of apples in the summer.
Because you cannot use the present continuous tense (present form of the verb “be” + the present participle of a verb) to describe events that happen normally, or for a long time. E.g., you can say "I am playing rugby on Saturday afternoon" as it is happening once on one Saturday afternoon only. However, you cannot say "I am playing rugby on Saturday afternoons," as it's an ongoing event that happens normally on ongoing Saturdays. Instead, you have to say "I play rugby on Saturday afternoons." (native British English speaker)
No. It is 'a lot of apples'. Maybe this will help:
Comemos mucho = We eat a lot ('a lot' is describing how much we eat)
Now compare that to Duo's sentence where we are still eating but instead of talking about the volume that we eat overall we are talking specifically about the volume of apples we eat. Did that help?
We tend to use on for days of the week but not for months or seasons. So, on Tuesday but in February or in spring. As a British English speaker I use the form at the weekend/at weekends; however, I believe that US English speakers are more likely to use on the weekend/on weekends.
When used as an adjective 'mucho' must match the gender and number of the noun it describes.
mucha vida = a lot of life (vida is feminine and singular)
mucho músculo = a lot of muscle (músculo is masculine and singular)
muchas cosas = many things (cosas is feminine and plural)
mochos libros = many books (libros is masculine and plural)
In Duo's sentence it is talking about a lot of apples. The word 'manzanas' is feminine and plural. So 'mucho' changes to 'muchas'.
The word 'mucho' can also be used as a adverb. In that case it never changes form from 'mucho'. You can tell from the context of the sentence if the word is an adjective or an adverb by asking yourself 'what is there a lot of?'. If the word is describing the verb it is the adverb and doesn't change. If it if describing a noun it is an adjective and must match it.
Encanto mucho los libros. = I love books a lot.
Encanto mochos libros. = I love a lot of books.
Notice in the first sentence there is a lot of 'love'. So 'mucho' is describing the verb making it an adverb and doesn't change form. In the second example there is a lot of 'books'. So 'mucho' is describing the noun 'libros' which is masculine and plural.
Did that help?