になる・くなるTo become (Adjective)

になる (with な-Adjectives), or くなる (with い-Adjectives) is an expression used in Japanese to show that something is 'becoming (A)'. It can be used with any form of なる, to highlight that something will become (non-past), is becoming (continuous), or has become (past). It can even be used with negative forms, to show that something 'will not become (A)'.

In the example with the tea getting cold, we can see how ている sometimes highlights something that exists in an ongoing state, rather than changing any further. In other words, 'the tea has gotten cold, and remains to be cold'.

Additionally, the adjective いい 'good' is an exception to the regular conjugation rules for い-Adjectives, and will conjugate as よくなる, not いくなる.