Learn Dutch: Hebben jullie het oude huis alleen behangen? - Did you guys wallpaper the old house alone?

The word alleen means "alone" in the sense of not having any company. So this sentence asks if you guys were the only people in the house doing the work. If you wanted to ask "Did you do it yourselves?" (as opposed to hiring a professional decorator), you would use zelf.

Why does oude get an -e added to the end?

The word huis is a het word. When you use the definite article het (the) before an adjective, the adjective always gets an -e ending. If it were "an old house" with the indefinite article een, the -e would be dropped (een oud huis).

Why doesn't the past participle behangen start with ge-?

Verbs that start with inseparable prefixes like be-, ge-, ver-, or ont- never get an extra ge- in their past participle form. Because the verb behangen (to wallpaper) starts with be-, its past participle is just behangen.

Can I put alleen at the very end of the sentence?