Learn Italian: Questi stivali non sono comodi. - These boots are not comfortable.

You would use gli, making it gli stivali. Because stivali is a masculine plural noun starting with an 's' followed by a consonant (st-), it requires the special article gli instead of the usual i.

Why do the words questi, stivali, and comodi all end in the letter -i?

In Italian, describing words must match the gender and number of the noun. Stivali is a masculine plural noun. Therefore, the word for 'these' (questi) and the adjective 'comfortable' (comodi) both take the masculine plural ending -i to match it.

Why is non placed before sono?

In Italian, the negative word non (meaning 'not') always goes right before the verb. While in English you say 'are not', the Italian word order is literally 'not are' (non sono).

How would I say 'This boot is not comfortable' in the singular?