Haec culcita mollior est quam illa, itaque infans bene dormit.

Because culcita is a feminine singular noun, and haec is the feminine singular nominative form of the demonstrative hic, haec, hoc (this).

So:

• haec culcita = this mattress
• hic would be masculine
• hoc would be neuter

The demonstrative has to agree with the noun it describes in gender, number, and case.

Latin often leaves out a noun when it is easy to understand from the context.

So quam illa literally means than that one, where that one really means that mattress.

The full version could be:

• Haec culcita mollior est quam illa culcita.

But Latin normally does not need to repeat culcita if the meaning is already clear.

Mollior is the comparative form of mollis (soft).

So:

• mollis = soft
• mollior = softer

In this sentence, mollior agrees with culcita, so it is nominative singular feminine. But note that the comparative ending -ior is the same for masculine and feminine nominative singular, so the form itself is not uniquely feminine.

Quam is the normal word used with a comparative adjective to mean than.

So:

• mollior quam illa = softer than that one

This is one of the most common comparison patterns in Latin:

• comparative adjective + quam

For example:

• maior quam = bigger than
• celerior quam = faster than
• pulchrior quam = more beautiful than

Because they are being compared directly with quam, and both refer to the thing being described as soft.

In other words:

• haec culcita = this mattress
• illa = that one / that mattress

Both are understood as the subjects of the comparison, so both appear in the nominative.

Latin often can leave out forms of to be when the meaning is obvious, especially in short expressions. But in a normal full sentence like this, est is very natural and standard.

So:

• Haec culcita mollior est quam illa = completely normal
• Haec culcita mollior quam illa = possible in some contexts, but less basic and less expected for learners

Including est makes the sentence clearer and more straightforward.

Itaque means and so, therefore, or so.

It connects the first idea to the second:

• This mattress is softer than that one, so the baby sleeps well.

It shows that the second clause is a result of the first.

Infans is a third-declension noun. Not all Latin nouns use first- or second-declension endings like -a or -us.

Here, infans is the nominative singular, and it means infant, baby, or child depending on context.

It is the subject of dormit, so nominative is exactly what we expect.

Because bene is an adverb, and adverbs modify verbs.

Here it modifies dormit (sleeps), so:

• bene dormit = sleeps well

By contrast:

• bonus / bona / bonum are adjective forms meaning good
• adjectives describe nouns, not verbs

So Latin needs the adverb bene, just as English uses well, not good, in sleeps well.

Because its subject, infans, is singular.

• infans = one baby
• dormit = he/she/it sleeps

If the subject were plural, the verb would be plural too:

• infantes bene dormiunt = the babies sleep well

Both are present indicative active.

• est = is
• dormit = sleeps / is sleeping

So the sentence describes a present situation:

• This mattress is softer than that one, so the baby sleeps well.

The word order is natural, but Latin word order is more flexible than English word order.

This sentence is:

• Haec culcita mollior est quam illa, itaque infans bene dormit.

A learner should notice that Latin does not depend as heavily on word order to show grammatical relationships, because endings do much of that work.

The chosen order is clear and idiomatic:

• Haec culcita introduces the topic
• mollior est quam illa gives the comparison
• itaque marks the result
• infans bene dormit gives the consequence

Other orders are possible, though not all would sound equally natural.

By itself, illa could mean that woman, that one, or something feminine singular in the right context. But here the context makes it clear that it refers back to culcita.

So in this sentence, illa means:

• that one
• that is, that mattress

This kind of omitted noun is very common in Latin.

The dictionary form is culcita, culcitae, a feminine first-declension noun.

That tells you:

• nominative singular: culcita
• genitive singular: culcitae
• gender: feminine

That is why words agreeing with it in this sentence are feminine, such as haec.

A useful breakdown is:

• Haec — this; feminine singular nominative
• culcita — mattress; feminine singular nominative
• mollior — softer; comparative adjective, nominative singular
• est — is
• quam — than
• illa — that one / that mattress; feminine singular nominative

So the structure is basically:

• This mattress is softer than that one.

Here is the breakdown:

• itaque — and so / therefore
• infans — baby / infant; nominative singular
• bene — well
• dormit — sleeps

So the structure is:

• Therefore the baby sleeps well.

This is a very standard pattern:

• connector
• subject
• adverb
• verb