Learn French: C'était une idée idiote.

We use the imperfect tense (l'imparfait) because the speaker is describing a past state or an ongoing opinion, rather than a single, completed action. You are setting the scene or giving your feeling about the idea in the past.

Since idée is a feminine word, why do we use c' instead of elle? Shouldn't it be elle était une idée idiote?

In French, when you use the verb être (to be) followed by a determiner (like un, une, le, la) and a noun, you must use ce (or c' before a vowel). You would only use elle était if it were followed by an adjective alone, for example: Elle était idiote.

Why does the adjective idiote come after the noun idée?

Most adjectives in French come after the noun they describe. Only a specific group of common, short adjectives (referring to Beauty, Age, Goodness, or Size, often remembered by the acronym BAGS) come before the noun. idiote doesn't fit into those categories, so it follows the normal rule and goes after idée.

Could I use the word bête instead of idiote here?