Learn Turkish: Adamlar büyük kutuları kamyonla taşıyor. - The men are carrying the big boxes by truck.

Kamyonla combines the word kamyon (truck) with the suffix -la. This suffix is the attached version of the word ile, which means "with" or "by means of". In Turkish, you use this ending to show the vehicle or tool you use to do an action.

Could I write kamyon ile as two separate words instead of kamyonla?

Yes, you can! Kamyon ile is grammatically correct and means the exact same thing. However, attaching it directly to the word as the suffix -la or -le is much more common in everyday Turkish.

Why is there an -ı at the end of kutuları?

That is the accusative case ending, which marks a specific direct object. Because the men are carrying specific boxes (meaning "the big boxes" rather than just "big boxes in general"), we must add the -ı ending to kutular (boxes).

Since the subject is plural (adamlar), shouldn't the verb be taşıyorlar instead of taşıyor?

It can be, but it does not have to be. In Turkish, when the subject refers to people and already has a plural ending (adamlar), adding the plural ending -lar to the verb is optional. Leaving it off and saying taşıyor is very natural and common here.