Learn French: Je vois six grands arbres ici.

Because six comes right before a word starting with a consonant (grands), the final consonant sound is dropped. It is pronounced like "see" instead of "seess". You do this exact same trick with huit (eight) and dix (ten) when they are followed by a consonant!

I thought French adjectives usually come after the noun. Why is it grands arbres instead of arbres grands?

You are right that most adjectives come after the noun! However, a small group of common, short adjectives go before the noun. You can remember them with the acronym BAGS: Beauty, Age, Goodness, and Size. Since grand (big or tall) describes Size, it goes before the noun.

Why does grands have an "s" at the end?

In French, an adjective must match the noun it describes. Since there are six trees, the noun arbres is plural. Therefore, we have to add an "s" to the adjective grand to make it plural as well.

Do I pronounce the "s" at the end of grands?

Yes, but it makes a "z" sound here! Normally, the final "s" on grands is silent. But because the next word (arbres) starts with a vowel, French speakers link the words together so the sentence flows smoothly. This is called a liaison, making grands arbres sound like "grahn-zarbr".