Purely for sound/pronunciation. It is using an inversion to ask a question (reversing order of subject and verb). But "grogne-il" is hard to say and/or not pleasing to the ear, so the 't' is added: grogne-t-il.
Maybe the most common example of this is the inversion of 'there are': il y a becomes y a-t-il. Much easier to say than y a-il!
why does he speak = pourquoi parle -t-il
why do they read = pourquoi lisent-ils
why are we stopped = pourquoi sommes nous arrêtés
what does he read = que lit-il
what do they eat = que mangent-ils
what do we do = que faisons-nous
to ask for questions in french the subject is often invert ( more formal ) especially for written
Thank you, but in your examples the subject is specified by the il/ils/nous etc., whereas in the Duolingo question, the subject (le chien) is already specified, so the 'il' seems redundant.
E.g. in English: "Why does he bark?" - ok "Why does the dog bark?" - ok "Why does the dog he bark?" - not ok
Is this something specific to the sentence or is it always required. Would "Pourquoi l'homme mange?" be correct or would it always need to be "Pourquoi l'homme mange-t-il?"
Many thanks for your help
Inversion is one of three ways to ask a question. But you can only invert a verb with a pronoun. It's illegal to say grogne-le chien. So you add in the pronoun (il). That gives you Le chien grogne-t-il? "Is the dog growling?"
The "-t-" is a little pronunciation assistance. It has no meaning. It isn't always used. Only use -t- with certain verbs, and only in the third person, which is il/elle.
Je ne sais pas "pourquoi le chien grogne" is correct I don't know why the dog is barking.
Pourquoi le chien grogne-t-il Why is the dog barking?
To turn it around, if you said in English "I don't know why is the dog barking" it sounds wrong.
It's not quite that bad in French, but adding -t-il improves the question.