There are two types of French "h": H muet (mute or silent) and H aspiré (aspirated or breathy). Despite the different names, they are both silent!
The difference (in Modern French) is whether they elide or not. H muet elides, so "le/la" + H muet becomes "l'h-", whereas "le/la" + H aspiré remains "le/la h-".
"hibou" uses H aspiré, which is why it's "le hibou".
P.S. There are also digraphs such as "ch" (pronounced "sh") and "ph" (pronounced "f"). These "h"s are neither meut nor aspiré because they combine with another letter to make a different sound. They are also no longer at the start of a word, so can't affect "le/la" anyway.
This is why you just have to internalize a language - any language - to really learn it. You can learn some rules to help you along, but ultimately their are too many rules and too many exceptions. You just have to practice, practice, practice!
Fortunately our brains are programmed to learn languages, so this is a doable task!
I suspect this is the reason: In some circumstances, "h acts like a consonant".
More specifically, Old French lost the Latin h, but got it again from Germanic influence. So it doesn't elide because the h really was a consonant.
...at least, mostly. It's more complicated since some words were altered to have a h aspiré or a h muet by analogy or dialectal variation, so you can't rely on etymology to tell you what's what.
When it's not threatening to stab me for forgetting to do my daily French lessons, at least.
This is an example of an "aspirated h," h aspiré. Certain words in French beginning in h do not form a liaison between the last vowel in the proceeding word and the first vowel sound in the next word. The h is still silent, but it acts like a consonant and it sounds like there is a brief pause between the two words rather than the smooth liaison. Le hibou sounds like "luh eboo." Unfortunately these words just have to be memorized. Dictionaries will have an asterisk next to an h aspiré.
Here are some more examples: le hockey, la Hollande, le héron, le héros. And a useful link
There is a little bit of information about "h aspiré" in the tips and notes as well under Basics 2
re mignon as cute. Doesn't mignon also mean darling? Some of the places where the translations has us say cute ie your daughters, baby animals, English speakers would be just as likely to say darling was cute.
lack of alternatives for cute! Not a favourite word in my vocabulary - has a rather childish ring to it. Only alternative I can find which is agreeable to Duolingo is sweet.
If you are using the word tiles, then there is only one correct answer offered. If you prefer to use another word you can use the keyboard function instead and type your answer.
FYI, depending on the situation, mignon can translate as sweet, cute, and sometimes nice.
I wrote "le hibou vert et mignon" and was counted incorrect.
It sounded like "le hibou vert et mignon" to me. How would that sound differently than "le hibou vert est mignon"?
I think you need to focus less on the difference in sound and more on the difference in context. "Le hibou vert et mignon" doesn't make sense as a sentence, or even a phrase, but "Le hibou vert est mignon" does. To be clear, "et" and "est" are pronounced almost identically, but they're used differently.