是 or 很

是 (shì) means 'to be', but only between a subject and a noun: 她是医生 (She is a doctor), 今天是星期五 (Today is Friday).

Adjectives work differently. In Chinese an adjective is a predicate on its own and takes no 是. Saying 我是累 for 'I am tired' is the classic beginner mistake. In a plain statement, put 很 (hěn) before the adjective instead: 我很累 (I am tired). Here 很 mostly links the subject and the adjective and hardly means 'very'. A bare adjective without 很 sounds like a comparison (今天冷，昨天不冷).

• noun (person, thing, place, day) → 是: 他是我的哥哥
• adjective (quality, feeling) → 很: 这本书很有意思

Test: does the word after the gap name a person, thing, place or day (医生, 中国人, 星期五)? Use 是. Does it describe a quality or feeling (累, 贵, 好吃)? Use 很.