Breakdown of Questi contenitori, li devi lavare bene.
questi
these (masculine)
bene
well
dovere
must
lavare
to wash
Questions & Answers about Questi contenitori, li devi lavare bene.
Why do we need the pronoun li when the sentence already states Questi contenitori?
This is the main feature of left dislocation. When you move a direct object to the beginning of a sentence to make it the main topic, Italian requires you to use a matching clitic pronoun before the verb to refer back to it. Since contenitori is masculine plural, we use the masculine plural pronoun li.
Could I just leave out the li? In English, it's fine to say 'These containers, you must wash well.'
No, in Italian it sounds grammatically incomplete to leave it out. Whenever you pull the direct object to the front of the sentence for emphasis, adding that resumptive pronoun is completely mandatory, not optional.
Since there is an infinitive verb (lavare), can I attach li to the end of it?
Absolutely. Because devi is a modal verb paired with an infinitive, you have a choice. You can put the pronoun before the conjugated verb (li devi lavare) or drop the final -e of the infinitive and attach the pronoun directly to it (devi lavarli). Both are perfectly correct and mean the exact same thing.
Why not just use the standard word order and say Devi lavare bene questi contenitori?