Yes, it is. But when we translate into English, which of the following would you be happiest with?
She was painting the barn with a spray gun, and when she passed he became red.
He had already been in the sun too long, and she noticed when she passed by he was sunburned (became red).
When she passed by, he immediately converted to Soviet communism (became red).
When she passed by, he blushed (became red).
Etc. We might imagine other contexts to explain his color change, but Duo doesn't give us any. The English phrase "became red" has too many possible senses, and we should choose the one that is most likely, and state it in the best English we can muster rather than word-for-word conversion.
Because the Italian doesn't use the word "arrosire" is not sufficient reason to reject a sensible translation. Going the other way, "He flew down the street" would not be best translated by "volare" but by "correre" even though the English doesn't use the verb "sped."
So do we in Croatian, and I said up there that it really would've been helpful, but once you analyse the sentence more carefully and realize that 'lei diventava rosso' just couldn't be a sentence because rosso is masculine, you see that there really is only one way the sentence could be understood. But it would be useful, that's for sure.
You are right that English "would" sometimes must be translated into the Italian conditional tense (il condizionale). For example, in a sentence like this:
"If I were rich, I would buy her a car."
However, use of "would" in English is not always conditional. That is, it does not always trigger a translation into the Italian conditional tense . For example, consider this English sentence:
"When the water was warm, we would swim for hours".
In the sentence above, the use of "would" does not signal the conditional but rather repetitive or customary action. The correct translation into Italian is therefore the imperfect (l'imperfetto dell'indicativo).
Edith, take a look at the other comments on this page. The second clause does not require the 'lui' to communicate that 'he' is becoming red, because the masculine ending on the adjective 'rosso' suffices for that.
On the other hand, the opening words, 'quando passava', do not by themselves determine whether their subject is male or female. That clause therefore cries out for a clarifying subject. So it is natural to interpret the pronoun that follows those words as the subject of the first clause, not as the subject of the second clause.
So the DuoLingo sentence with 'lei' means 'As she passed by, he became red'; and your suggested sentence means 'As he passed by, he became red'.
In speaking the Italian, there would be a pause between the two clauses, like this: Quando passava lei [pause] diventava rosso.
English does not make the same distinctions in verb tense as Italian does. One often uses the simple past in English to describe ongoing or habitual situations.
For example: "Whenever she passed by, he grew red." This is an habitual situation and so would be imperfetto in Italian, but in English the simple past here is quite idiomatic.
By translating into Italian I wrote "Quando lei passava lui diventava rosso." and this was accepted by Duo. But even though it seems grammatically and literally right, I guess that "Quando passava lei diventava rosso." Or even "Quando passava lei diventava rosso lui." sounds better for an italian ear. The last sentence is emphasising the lui, meaning he turned red while somebody else did not in the same situation.
6hxvEzF4, your analysis of the sentence is off. There is no "root verb". What we have here are two separate clauses, each with its own subject and its own conjugated verb: she passes by, he grows red. Note that exactly the same structure occurs in English as well.
I think what is confusing you is that in the Italian, the subject word "he" is not explicitly stated, whereas in English it is. The Italian does not need to state the pronoun "he" because the adjective "rosso" has a masculine ending that makes it clear that the unstated subject must be masculine.
Because the subject "he" of the second clause is not explicitly stated in the Italian, you may have failed to notice that a new clause has begun here -- not a continuation of the first clause.
In speaking, there would be a pause between the two clauses, like this: Quando passava lei [pause] diventava rosso.
In Italian, one way the imperfect tense is used is to express an action in the past that went on simultaneously with another action. Therefore, both verbs are conjugated in the imperfect. Another example (from Barron's 501 Italian Verbs), is "Mentre mia madre leggeva, mio padre guardava la TV."