Learn Arabic (Modern Standard): الشجرة كبيرة.

The letter ش (shiin) at the beginning of شجرة (tree) is a "Sun letter". When the definite article ال (al-) attaches to a word starting with a Sun letter, the "l" sound blends into it. Instead of pronouncing the "l", you double the ش sound, making it "ash-shajara".

Do we still write the "l" in Arabic even though we skip pronouncing it here?

Yes! Even though you pronounce it "ash-shajara", you must always write the full ال (al) prefix. The spelling remains الشجرة, keeping the ل (laam) clearly visible in the text.

Why does the adjective كبيرة (kabiira) end with the letter ة (taa marbuuta)?

Adjectives in Arabic must match the gender of the noun they describe. Because الشجرة (the tree) is a feminine word ending in a ة, you must add the feminine ending ة to the word كبير (big), making it كبيرة.

Why isn't there a word for "is" between "the tree" and "big"?

Arabic does not use a word for "is" or "are" in basic present tense sentences. By putting a definite noun like الشجرة (the tree) right next to an indefinite adjective like كبيرة (big), it automatically forms the complete sentence "The tree is big."