Haec sententia peior est quam illa.

Haec means this and is the feminine singular nominative form of hic, haec, hoc.

It matches sententia, which is a feminine singular noun, so Latin uses the feminine form of this:

• haec sententia = this sentence / this opinion / this statement

Because haec and sententia go together, they must agree in gender, number, and case.

Sententia is a first-declension feminine noun. Depending on context, it can mean things like:

• sentence
• opinion
• judgment
• statement
• thought

In this sentence, it is in the nominative singular because it is the subject of est.

So:

• haec sententia = this sentence / this statement
• illa refers to that one, with sententia understood

Because the comparative of malus (bad) is irregular.

The forms are:

• positive: malus = bad
• comparative: peior = worse
• superlative: pessimus = worst

So Latin does not say malior. It uses the irregular comparative peior.

In this sentence:

• peior est = is worse

Because peior here is agreeing with sententia, which is feminine singular.

The comparative adjective has these nominative singular forms:

• masculine/feminine: peior
• neuter: peius

So:

• haec sententia peior est = this sentence is worse
• if the noun were neuter, you would use peius

For example:

• hoc verbum peius est = this word is worse

Est means is and is the third person singular present active indicative of esse (to be).

It links the subject haec sententia with the predicate adjective peior:

• haec sententia = subject
• peior = predicate adjective
• est = linking verb

So literally:

• This sentence worse is than that one
• natural English: This sentence is worse than that one

In some Latin sentences, est can be omitted if it is obvious, but here it is expressed.

Quam means than after a comparative.

So:

• peior ... quam illa = worse than that one

This is the standard way Latin makes many comparisons:

• comparative adjective/adverb + quam

Examples:

• maior quam = greater than
• celerior quam = faster than
• peior quam = worse than

So quam introduces the second item in the comparison.

Because Latin often omits a noun when it is easily understood from the context.

Here, illa means:

• that one
• more fully: that sentence / that statement

So illa is standing in for illa sententia.

This is very natural in Latin, just as in English we can say:

• This sentence is worse than that one

instead of repeating:

• This sentence is worse than that sentence

It is nominative feminine singular, because it stands for illa sententia.

With comparative + quam, Latin usually puts the thing compared in the same case as the first item when quam is used.

So:

• haec sententia = nominative
• illa = nominative

This is because the full structure is understood as:

• Haec sententia peior est quam illa sententia est.

Even though the repeated words are omitted, the grammar remains the same.

Yes, sometimes Latin uses the ablative of comparison instead of quam.

For example, with a comparative adjective, Latin can say:

• Haec sententia illa peior est.

where illa would be in the ablative.

But with pronouns, and especially for clarity, quam is often very natural and easier for learners to recognize.

So the sentence with quam is a straightforward and standard way to say it:

• Haec sententia peior est quam illa.

Latin word order is much more flexible than English word order because Latin uses endings to show grammatical function.

English depends heavily on position:

• This sentence is worse than that one

Latin can move words around more freely because forms like haec, sententia, peior, and illa show their roles through their endings.

The given order is perfectly normal:

• Haec sententia peior est quam illa.

But other orders could also be possible, depending on emphasis, for example:

• Peior est haec sententia quam illa.
• Haec peior est sententia quam illa.

The original order is simple and clear for learners.

A helpful breakdown is:

• haec = this
• sententia = sentence / statement / opinion
• peior = worse
• est = is
• quam = than
• illa = that one

So the structure is:

• This statement is worse than that one

Or more literally:

• This statement worse is than that one

Seeing the agreement can also help:

• haec sententia = feminine singular subject
• peior = feminine singular comparative adjective
• illa = feminine singular demonstrative standing for the omitted noun

That is the main grammatical pattern of the sentence.