"El niño está enfermo."
Translation:The boy is sick.
We use "is" in a number of very different ways in English. Spanish uses different words for some of those uses. In this case "está" is from the verb "estar" and "es" is from the verb "ser". I use the following mnemonic to help remember where to use each but it still a bit confusing. Ser: Description, Occupation, Characteristic,Time,Origin, Relationship(DOCTOR) and Estar: Position, Location, Action, Condition, Emotion(PLACE). In this case it is a condition and that requires '"estar".
Be sure to notice that it is "está" and not "esta"!
Esta: this / that
Está: third person singular conjugation of the verb "estar" which means "to be" when talking about location (Ella está en Madrid - She is in Madrid) or about properties that are easily changed (El está cansado - He is sleepy).