Learn Arabic (Modern Standard): هي مريضة.

In Arabic, simple present-tense sentences don't use a linking word for "is", "am", or "are". You just put the subject (هي) and the description (مريضة) side by side to mean "She is sick."

Why do we use the full word هي instead of a pronoun suffix attached to the end of a word?

هي (hiya) is an independent subject pronoun, just like "she" in English. You use it when the pronoun is the main doer or subject of the sentence. Pronoun suffixes are only used when the pronoun is an object (like "saw her") or shows possession (like "her book").

Why does the word for sick, مريضة, end with the letter ة (taa marbuutah)?

Adjectives in Arabic must match the gender of the person they describe. Since the subject here is هي (she), which is feminine, we must add the feminine ending ة to the base masculine word مريض (sick) to make it مريضة.

I keep mixing up the words for "he" and "she". How can I remember هي?

This is a very common beginner mix-up! هي (hiya) means "she", and هو (huwa) means "he". A helpful trick is remembering that the Arabic word for "she" (هي) sounds a lot like the English word "he" — the exact opposite of its meaning!