Il cuscino è rotondo.

In Italian, il is the singular, masculine definite article used before most consonant-initial nouns.
• cuscino begins with a consonant (c), so it takes il.
• If the noun were feminine, you’d use la (e.g. la sedia).
• For masculine nouns starting with s + consonant, z, gn, ps, etc., you’d use lo (e.g. lo studente, lo zio).

Many Italian nouns ending in -o are masculine, while those ending in -a are often feminine. This is a general pattern:
• -o → masculine (singular)
• -a → feminine (singular)
• -i → masculine (plural)
• -e → feminine (plural)
There are exceptions, so it’s best to memorize each noun with its article: il cuscino, la mano, etc.

Italian adjectives must agree in gender and number with the noun they modify. Here, rotondo is:
• Masculine (because cuscino is masculine)
• Singular (we’re talking about one pillow)
If you had a feminine or plural noun, the ending would change:
• Feminine singular: rotonda
• Masculine plural: rotondi
• Feminine plural: rotonde

Both subject and adjective must become plural and the verb changes accordingly:
• I cuscini sono rotondi.
– I is the plural masculine definite article.
– sono is the 3rd-person plural of essere (to be).
– rotondi is masculine plural.

In Italian, the default (neutral) position for most adjectives is after the noun:
• Il cuscino è rotondo.
Putting adjectives before the noun can add emphasis or change nuance, but following the noun is the standard descriptive order.

• Rotondo describes something with a rounded, ball-like or spherical shape (3D).
• Circolare refers to something in the shape of a circle (2D).
A round table top might be circolare, while a ball or pillow is rotondo.

• È with a grave accent is the 3rd-person singular present of essere (to be).
• E without an accent is the conjunction “and.”
The accent is mandatory to distinguish them, so you cannot omit it in è.

• cuscino: /kuˈʃiːno/
– c before u = /k/
– sc before i = /ʃ/ (like English “sh”)
– Stress on the second syllable: cu-SCU-i-no
• rotondo: /roˈtondo/
– r is rolled or tapped
– Stress on the second syllable: ro-TON-do

Yes, but there’s a nuance:
• Il cuscino rotondo is an attributive adjective (“the round pillow,” identifying which pillow).
• Il cuscino è rotondo is a predicative adjective (a statement: “the pillow is round”).
Use the copula è when making a comment or describing a state.