Haec res facilis videtur, sed difficilior est quam illa.

Because haec has to agree with res.

• res is a feminine singular noun
• the subject of the clause is haec res
• so the demonstrative this must also be feminine singular nominative

The paradigm is:

• hic = masculine
• haec = feminine
• hoc = neuter

So haec res means this thing / this matter.

Res is a very common Latin noun meaning thing, matter, affair, or situation, depending on context.

It is a fifth-declension noun, and it is normally feminine. So here:

• res = nominative singular
• feminine
• subject of videtur

A native English speaker often expects thing to be neuter, but Latin nouns have grammatical gender, not biological gender. So a word meaning thing can still be feminine.

Because facilis is an adjective describing res, and it must agree with res.

Here res is:

• feminine
• singular
• nominative

So the adjective must also be nominative singular feminine. For facilis, the masculine and feminine nominative singular are the same: facilis.

By contrast, facile would be:

• the neuter nominative/accusative singular form, or
• an adverbial-looking form that does not fit here as the predicate adjective for res

So haec res facilis videtur = this thing seems easy.

Because facilis is a third-declension adjective of two terminations.

Its basic forms are:

• facilis for masculine and feminine
• facile for neuter

So the feminine form is not something like a first-declension -a form. Third-declension adjectives do not work that way.

That is why both vir facilis and res facilis are possible:

• vir facilis = an easy/easygoing man
• res facilis = an easy thing

Yes: formally, videtur is the passive form of video.

But Latin very often uses videor, videri with the meaning seem or appear.

So:

• videt = sees
• videtur = is seen or, very commonly, seems

In this sentence, haec res facilis videtur is a standard Latin way to say this thing seems easy.

Also notice that facilis stays in the nominative, because it is a predicate adjective linked to the subject res.

Because the meanings are slightly different.

• facilis videtur = seems easy
• difficilior est = is more difficult

So the sentence contrasts appearance in the first clause with reality/comparison in the second.

Latin could have said facilis est if it wanted to say is easy. But videtur adds the idea of it appears easy.

Difficilior is the comparative form of difficilis.

• difficilis = difficult
• difficilior = more difficult

For many Latin adjectives, the comparative is formed with:

• -ior for masculine/feminine
• -ius for neuter

Since res is feminine singular, the correct form here is difficilior.

So:

• res difficilis = a difficult thing
• res difficilior = a more difficult thing

Because quam is the normal word for than after a comparative.

So:

• difficilior quam illa = more difficult than that

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

• adjective in the comparative + quam + same case

For example:

• maior quam frater = bigger than the brother
• clarior quam luna = brighter than the moon

Here the same pattern is used with difficilior.

Because illa is being compared with haec res, and with quam Latin normally puts the second item in the same case as the first.

The first item is the subject:

• haec res = nominative singular

So the second item is also nominative:

• illa = that one, understood as illa res

If the noun were written out fully, it would be:

• difficilior est quam illa res

not quam illam rem.

Because Latin very often leaves out a noun when it is obvious from context.

Here illa means:

• that one
• literally that [thing]

The omitted noun is res, understood from the first clause.

So:

• haec res = this thing
• illa = that one = that thing

English does the same thing: we often say this one and that one without repeating the noun.

Not as much as it would be in English.

Latin word order is relatively flexible because the endings show the grammatical relationships. This sentence is arranged in a very natural prose order:

• Haec res = topic first
• facilis = predicate adjective
• videtur = verb at the end of the clause
• sed introduces the contrast
• difficilior est quam illa gives the comparison

A different order could still be grammatical, for example:

• Haec res videtur facilis, sed est difficilior quam illa.

That said, the original order is smoother and more idiomatic.

It may sound a little surprising at first, but it is perfectly sensible.

The idea is not necessarily:

• this is difficult

It is more like:

• this looks easy
• but compared with that other thing, it is harder

So both parts can be true at once:

• something can seem easy
• yet still be more difficult than another thing

This is a meaning question rather than a grammar problem, but it is a very natural thing for learners to wonder about.