Learn French: En effet, ces vieux casse-noix sont très bizarres. - Indeed, these old nutcrackers are very weird.

Casse-noix is a compound noun made of a verb (casse, to break) and a noun (noix, nut). In verb-noun compounds, the verb part never changes in the plural. The noun part can become plural, but since noix already ends in an x in the singular, it doesn't change either. So, it is un casse-noix in the singular and des casse-noix in the plural.

I always mix up en effet and en fait. How do they differ here?

En effet means indeed or that is right. It is used to confirm, explain, or agree with something just said. On the other hand, en fait means in fact or actually, and is mostly used to contradict or correct an assumption. Here, the speaker is agreeing with a previous statement that the nutcrackers are weird.

Why doesn't vieux have a plural marker?

Words that already end in -s, -x, or -z in the singular do not take an extra letter in the masculine plural. Because the singular masculine form is already vieux, the plural remains vieux.

Why is vieux placed before casse-noix rather than after?