If you don't have the umlauted ä, ö, or ü available, substitute ae, oe, or ue. I have seen samples of older style cursive manuscript wherein the letter 'e' is represented by a 'rune' of 2 short parallel strokes. In places we would today expect " over a vowel, the 2-stroke 'rune' follows the vowel. I'm guessing the strokes were moved over the affected vowel & changed to dots & die Umlautpunkte (the around-sound-[pen]pricks) became an "umlaut."
I think "das Gebäude" is the better/more common way of referring to a building, edifice, or structure, whereas "der Bau" is more like active construction or an architectural site. "Der Bau" is also refers to a burrow (i.e., for an animal) or (Umgangssprach) jail.
WRT the plural, per the Wiktionary:
Nowadays, the plural Bauten is used in sense 1, meaning "buildings", while the plural Baue is used for sense 3, meaning "burrows".
The whole thing reminds me of the witticism: "Why do we say we live in buildings? Aren't we really, for the most part, living in 'builts'?"
There have been several hiccoughs in this section. For the sentence "Die Gebäude sind groß," I entered the answer "The buildings are big." Duo counted this as wrong and gave the correct answer as "The buildings are big." :-/
I did report this, but I thought I'd mention it here in case anyone else was having trouble.
That is wrong.
First, Gebäude--like all nouns in German--should be capitalized.
Second, a is not the same as ä, but die Eule is very forgiving (i.e., he ignores entirely) about the use of umlauts. (Well, not entirely. The feedback on any challenge will note "Pay attention to the accents.")
Third, die Eule is pretty forgiving about typos. So long as you're one or two letters off, if the mistake does not create another valid word, (e.g., "then" when you mean "they" or "lieben" when you mean "leben"), then you will usually be counted as correct (usually with a message in the feedback noting that you made a typo).
"large" and "big" both translate to groß.
When you are unsure why an answer was not accepted, please make a screenshot that shows the question and your entire answer. Then upload thte screenshot to a website somewhere such as imgur or postimage, please, and post the URL of the image in a comment here.
That way, people can give more precise help.
For example, you might have had a listening exercise ("type what you hear") but instead you translated into English.
Or you might have made a spelling mistake somewhere else in the sentence.
We can't see any of that if you only talk about one word at a time.
Please show us what happened when you ask for help.