Learn Dutch: Het slot van de deur zal wel kapot zijn. - The lock of the door is probably broken.

While zullen (here as zal) is typically used for the future tense to mean "will", Dutch also uses it to express an assumption or strong probability. When you use zal to describe a present situation, it translates to "must be" or "is probably".

What is the purpose of the word wel in this sentence?

The particle wel frequently pairs with zullen to soften an assumption. Together, zal wel is a very common and natural way to say "is probably" or "I assume that". Without wel, the sentence would sound more like a strict prediction about the future.

Why is the verb zijn (to be) at the very end of the sentence?

Because zal is a modal verb, it takes the standard second position in the sentence and pushes the main infinitive verb—in this case, zijn—to the very end of the clause. This is a standard Dutch word order rule you will see with all modal verbs.

Could I use the word gebroken instead of kapot for a broken lock?

No, kapot is the correct choice here. Dutch typically uses gebroken for things that physically snap or shatter into pieces, like a bone or a glass window. For mechanical objects, devices, or systems that simply stop working (like a lock), you always use kapot.