Litterae Graecae mihi gratiores sunt quam litterae Latinae.

They are adjectives agreeing with litterae. In Latin, adjectives must match the nouns they describe in gender, number, and case.

Here, litterae is:

• feminine
• plural
• nominative

So the adjectives must also be feminine plural nominative:

• Graecae
• Latinae

Because it also agrees with litterae, which is plural. The basic comparative form is gratior for singular, but the nominative plural masculine/feminine form is gratiores.

So:

• littera gratior = a letter is more pleasing
• litterae gratiores = letters are more pleasing

It is the comparative adjective of gratus.

Gratus can mean things like:

• pleasing
• welcome
• dear
• agreeable

Its comparative means more pleasing or dearer. So gratiores means more pleasing because it is agreeing with a plural noun.

Because gratus and its comparative are commonly used with the dative of the person affected.

So:

• mihi gratus est = it is pleasing to me
• mihi gratiores sunt = they are more pleasing to me

A native English speaker often expects to me as a separate prepositional phrase, but Latin usually expresses that idea with the dative case alone.

Quam introduces the second part of the comparison: than.

So the structure is:

• gratiores ... quam ... = more pleasing ... than ...

It connects litterae Graecae with litterae Latinae in a comparison.

Because after quam, Latin normally puts the second thing compared in the same case as the first.

Here the first item compared is litterae Graecae, which is nominative, so the second item is also nominative:

• quam litterae Latinae

Latin also has another way to make comparisons: the ablative of comparison, without quam:

• Litterae Graecae mihi gratiores sunt litteris Latinis.

That means the same thing.

Because Latin often leaves out repeated words when they are easy to understand from the context.

The idea is basically:

• Greek letters are more pleasing to me than Latin letters are.

Latin does not need to repeat the second sunt. The reader can supply it mentally.

Because Latin word order is much more flexible than English word order. The endings tell you what each word is doing, so Latin does not depend as heavily on position.

This sentence could be arranged differently without changing the basic meaning. The given order is natural Latin, and putting mihi before gratiores sunt nicely keeps the idea to me close to more pleasing.

No. That is a very common confusion.

In English, grateful usually describes a person's feeling of thankfulness. But Latin gratus often means pleasing, welcome, or dear.

So here gratiores sunt mihi does not mean are more grateful to me. It means are more pleasing to me or I like them better.

Because Latin has no definite article. It does not have separate words for the or a/an.

So litterae Graecae can mean:

• Greek letters
• the Greek letters
• sometimes even Greek literature

The exact English translation depends on the context.