Learn Spanish (Spain): Anduvimos mucho, pero el plan fracasó.

Andar (to walk) has a highly irregular stem in the preterite tense. Instead of keeping the regular and- stem, it changes to the irregular u-stem anduv-. Because of this, it does not use the regular past tense form andamos.

Isn't andar an -ar verb? Why does it end in -imos, which is usually for -er and -ir verbs?

Good catch! Verbs with these special irregular stems in the preterite (like the anduv- stem) share a unique set of endings. For the nosotros (we) form, this special ending is always -imos, even if the original verb ends in -ar.

Why does fracasó have an accent mark on the final o?

Fracasar (to fail) is a regular -ar verb. In the preterite tense, the it ending for regular -ar verbs is -ó. The accent mark is crucial because it tells you to stress the final syllable, which distinguishes this past tense form (fracasó, meaning it failed) from the present tense form fracaso (meaning I fail).

Could mucho change to muchos in a sentence like this?

Not in this case! Here, mucho is an adverb describing the verb anduvimos (how much we walked). Adverbs never change their gender or number, so it stays as mucho. It would only change to muchos or mucha if it were an adjective describing a noun, like in muchos planes (many plans).