ok...so far "A" in hungarian has been translated into "the" in english. suddenly it is not and without an explanation. Self learning is not so easy
Not sure what chapter this sentence is in, but this point is explained in the notes to Adjectives 1: "You will come across so-called generic statements. Those are sentences that express something that is true in general [...] There is an important difference between such statements in English and Hungarian. While in English you don't have to use an article for the subject in such sentences, in Hungarian you usually do."
But how do you know whether the sentence is a so-called generic statement or not? This one is 'A regi angol autok...' and translated as Old English cars, without the article in English. But the previous sentence, 'A telefonok konnyuek...' seems just as generic about phones but the only correct translation was The phones are light, with the definite article. How do you know which they mean?
In general, you can't, unless context makes it very clear, and so both translations should be accepted - and if they aren't, report report report!
Why is adjective "guorsak" decline, but "régi" and angol stay in singular nominative?
In Hungarian, adjectives used as determiners of a noun are not declined to match the number of the noun: "A gyors autók" (The fast cars).
When they are used as predicates, they are: "Az autók gyorsak" (The cars are fast).