Lo sbadiglio del bambino è grande.

Because sbadiglio is a masculine singular noun that begins with s + consonant (sb-). In Italian, masculine singular nouns usually take il, but they take lo before certain sounds, including:

• s + consonant: lo studente, lo sport
• z: lo zaino
• ps: lo psicologo
• gn: lo gnomo

So it has to be lo sbadiglio.

Del is a contraction of di + il.

• di = of
• il = the
• del = of the

So del bambino literally means of the child or of the boy, and in natural English it often becomes possession: the child's.

That is why Lo sbadiglio del bambino corresponds to The child's yawn.

Italian does not form possession with an apostrophe the way English does. Instead, it usually uses:

• di
  • noun
• or a possessive adjective such as suo

So English the child's yawn becomes Italian lo sbadiglio del bambino.

A very literal breakdown is:

• lo sbadiglio = the yawn
• del bambino = of the child

Grande describes lo sbadiglio, not bambino.

The subject of the sentence is Lo sbadiglio del bambino. The core noun in that subject is sbadiglio.
Del bambino is just extra information telling you whose yawn it is.

So the sentence means that the yawn is big, not that the child is big.

Because grande is one of those Italian adjectives that has the same form for masculine and feminine singular.

So you get:

• un bambino grande = a big boy/child
• una casa grande = a big house

In the plural, it becomes grandi:

• bambini grandi
• case grandi

So with singular sbadiglio, grande is exactly the correct form.

Because here grande is a predicate adjective, used after the verb essere (to be).

Structure:

• Lo sbadiglio del bambino = subject
• è = is
• grande = big

So the pattern is just like English:

• The yawn is big
• Lo sbadiglio è grande

This is not the same as an adjective placed directly before or after a noun, as in un grande sbadiglio or uno sbadiglio grande.

The accent distinguishes è (is) from e (and).

• è = is
• e = and

So:

• Il bambino è stanco = The child is tired
• Il bambino e la mamma = The child and the mother

The accent is required in standard writing.

Yes, that is grammatically possible, but it means His/Her/Its/Your-formal yawn is big, so it can be less clear.

Del bambino is more specific because it clearly means the child's or of the child.

Compare:

• Lo sbadiglio del bambino è grande = The child's yawn is big
• Il suo sbadiglio è grande = His/Her yawn is big

So del bambino is often better when you want to avoid ambiguity.

Yes, it is grammatical and understandable. It means the yawn is big or large.

In real Italian, a speaker might also choose other adjectives depending on the nuance, such as:

• enorme = enormous
• lungo = long
• forte = strong/intense, in some contexts

But grande is perfectly fine for learning and for basic description.

A rough English approximation is zbah-DEEL-lyo.

A few useful points:

• the initial s in sb- often sounds like a z to English ears
• gli here is the Italian sound heard in words like famiglia
• that gli sound is not a normal English gl

You do not need a perfect imitation right away, but it helps to remember that sbadiglio is not pronounced with a hard g sound like in giggle.