Learn Arabic (Modern Standard): الشجرة طويلة والزهرة صغيرة. - The tree is tall and the flower is small.

In Arabic, you don't need a word for 'is' in the present tense. Instead, you create a 'nominal sentence' simply by placing a definite noun like الشجرة (the tree) right before an indefinite describing word like طويلة (tall). That contrast automatically creates the meaning 'The tree is tall.'

How do we know that طويلة (tall) and صغيرة (small) are indefinite?

You can tell they are indefinite because they do not have the definite article الـ (al-) attached to the beginning. When spoken with full grammar endings, they also end with a double-vowel 'un' sound called Tanween, which is the standard marker for an indefinite word.

What would happen if I put الـ (al-) on the adjectives too?

If you wrote الشجرة الطويلة (making both words definite), it would no longer be a complete sentence. It would just be a matching descriptive phrase meaning 'the tall tree'. Keeping the first word definite and the second word indefinite is the specific grammar rule needed to build a complete sentence here.

Why do the words for 'tall' and 'small' both end with the letter ة?

That letter is the Ta' Marbuta (ة), which typically marks a word as feminine. Because the subjects الشجرة (the tree) and الزهرة (the flower) are feminine nouns, the adjectives describing them must match in gender. Therefore, we add the Ta' Marbuta to the end of the adjectives.