Until now, I thought 新 meant 'new' in all contexts. In class recently, our tutor showed a slide in which he used 生词 for 'new words'. My dictionary gives the meaning of 生 when used as an adjective as 'raw, uncooked, unripe'. Is there some other subtle meaning of 生 when used in this context to distinguish it from 新 - when would I choose one over the over?
I am a Chinese.
新 means new and 生 means unfamiliar.
In your daily study, a word you first see could be either 新词 (new word) or 生词 (unfamiliar word) to you.
However, in public articles, 新词 usually stand for newly made word (ABSOLUTELY NEW TO EVERYONE), and 生词 stand for unfamiliar word (RELATIVELY NEW TO SOMEONE).
Hope this helps.
Try a dictionary such as the excellent 汉典.
生 is both a common and ancient character, so it has many meanings (汉典 lists 20), although a lot of them are related. The definition you are after is this one, which means "unfamiliar":
This is subtly different from 新 which means "new".
新词 = new words that did not exist
生词 = words new to me (literally raw/uncooked words)
They are as such because the opposition of 新(new) is 老(old)/旧(used), while for 生(raw/living), it is 熟(fully cooked). In French, also, they distinguish 'neuf' and 'nouvel' just so.
Generally we make metaphor of the process of learning things as raffinated cooking. For example, a stranger is 生人, however, a familiar person is 熟人 (literally cooked people, here should be taken as 'a learned people'). Although sometimes we regard this 熟 as abbr of 熟悉 (literally all-cooked-ly known)， it cannot really jump out of this opposition.
By the way, 新人 means a nova/noob（=新手）, its opposition is 老人/熟手 rather than 旧人.
And, 炒冷饭 means to learn something you have known well over and over.
新: new 生: fresh. There are many meanings of this character also. Fresh is only one of them.