Article indéfini Bonjour ! Je sais qu'on utilise les indéfinis quand il y a un adjectif près de nom. Mais dans cet exemple-ci: Je l'ai fait par une curiosité de savant. Est-ce qu'on peut expliquer l'utilisation de "une" comme ça ? Les mots "de savant" dans ce contexte ont un rôle d'un adjectif ?

Are or is "Every second there are/is about three humans more on the Earth." Which one shall I put?

How do we use "whether" Can we say "Everyone expressed doubt that he was hijacking" instead of "Everyone expressed doubt whether he was hijacking "? Do these sentences have the same meaning?

Each vs every What is the difference between these two: Each man of the crew was rewarded. Every man in the regiment displayed bravery. (Every man in the crew was rewarded. - can we build such a sentence?)

grazing / that graze Which of the following sentences is correct: A former director of the CIA said that the surveillance capabilities of US satellites would enable a skilled photographic interpreter to distinguish between Guernsey and Hereford cows grazing in a meadow. A former director of the CIA said that the surveillance capabilities of US satellites would enable a skilled photographic interpreter to distinguish between Guernsey and Hereford cows that graze in a meadow. Which one of them is more concise?

Time clauses When you have been here for another week you'll find it quite entertaining. When you are here for another week you'll find it quite entertaining. Which one is correct? I assume, that talking about future in when-clause we use present simple --> future simple template. Am I right?

Prepositions in/at I need to insert either in or at in this sentence: "We arrived at the airport ... good time for the plane." Considering a common phrase "in the right place at the right time", there should be "at" in the gap. Am I right?

Auxiliary verbs If I'm 100% sure that Tom knows something, how do I say: Tom will know it. Tom must know it. And also if Tom PROBABLY knows, what's better: Tom may know it. Tom should know it.

Why do we say... "I will be back at 8.30" Why don't we use present continuous (I'm being back at 8.30) ? It's an arrangement, right? Or is it just because it sounds weird? :)