Learn German: Ich knete den Teig, bis er weich ist. - I knead the dough until it is soft.

The word bis (until) is a subordinating conjunction. This means it introduces a dependent clause and forces the conjugated verb (ist) to move to the very end of that clause.

Why does it say bis er weich ist instead of bis es weich ist for "until it is soft"?

In German, pronouns must match the grammatical gender of the noun they replace. The word for dough is masculine (der Teig), so you must use the masculine pronoun er to refer back to it, even though we just say "it" in English.

Why is it den Teig and not der Teig in the first part of the sentence?

Teig is the direct object of the verb kneten (to knead) because it is the thing being kneaded. This puts it in the accusative case, which changes the masculine word for "the" from der to den.

Does bis always push the verb to the end of the sentence?

It does when it connects two full clauses with subjects and verbs, like it does here. However, bis can also be used simply as a preposition before a time expression (like bis morgen for "until tomorrow"), and in those cases, it does not affect the verb order.