“Grande” y “pequeño” son antónimos muy fáciles de recordar.

Because the subject is made of two items: grande y pequeño. In Spanish, two things joined by y normally take a plural verb, so you use son.

If there were only one item, you would use es: Grande es un adjetivo.

Because the sentence is talking about the words themselves, not using them to describe a noun. When Spanish mentions a word as a word, it often uses its basic dictionary form.

So grande and pequeño stay singular here.

Also:

• grande is the same in masculine and feminine singular
• pequeño is specifically the masculine singular form

If you wanted to talk about another form, you would name that form directly, such as pequeña or pequeños.

After ser, Spanish often leaves out the article when it is simply classifying something.

So:

• son antónimos = they are antonyms

This is similar to other patterns like:

• es profesor
• somos amigos

If you added los, it would usually sound more specific, as if you meant the antonyms in a particular context.

Because it describes antónimos, and antónimos is plural. Spanish adjectives normally agree with the noun they describe in number.

So:

• antónimo fácil
• antónimos fáciles

Because fácil de + infinitive is a very common Spanish pattern. It means easy to ...

So:

• fáciles de recordar = easy to remember

You will see the same structure in:

• difícil de entender = hard to understand
• fácil de usar = easy to use
• rápido de preparar = quick to prepare

Muy means very, and it modifies fáciles.

So:

• fáciles = easy
• muy fáciles = very easy

Like English very, muy does not change form.

The accent marks show which syllable is stressed.

• fáciles is stressed on fá: FÁ-ci-les
• antónimos is stressed on tó: an-TÓ-ni-mos

In antónimos, the stress is on the third-to-last syllable, and words of that type always take a written accent in Spanish.

Ñ is a separate Spanish letter. It sounds roughly like ny in canyon.

So pequeño is approximately: pe-KE-nyo

Also, qu before e gives a hard k sound, so peque- sounds like peke-.

Because the sentence is mentioning the words themselves, not using them normally as adjectives. Quotation marks help show that the writer is talking about the words as vocabulary items.

Spanish also often uses italics for this purpose.