天真 = naive/innocent 单纯 = simple and pure

When describing a person, "天真" means this person's outlook of the world is simple, inexperienced, not sophisticated. "单纯" would mean this person's interaction with people is in such a way that is not calculating, not suspecting. It's a word you wouldn't use to describe a schemer.

Good question. For me, they are similar but used a bit differently nowadays at least. I would like to offer my understanding. 天真 has a positive connotation if used to describe a child which focus on 真 true to yourself (e.g. 孩子天真的笑脸）, however used on adult implies naive in a negative way. For example we say someone has 天真的想法／做法 which implies this person is unrealistic and not mature. But when we describe someone with 单纯, we focus on 纯 pure in nature. We talk about someone 单纯 is normally positive means someone simple and pure. It's not simple minded but also has a hint of cleanness in it. We can look into the two characters separately for your understanding: 1. 单, 简单， 单身， 单人床 all says single 2. 纯， 纯洁，纯净，纯粹 all about purity. Beside talking about someone 单纯, we can also say 单纯的生活 Hope this helps. I am just a native speaker not a teacher. :)

I think it is not too much different between them especially daily life. Both them could describe people who are innocent, believing the world is always good, aren't tactful to others (in these context, both words is commendatory). But sometimes we use these words sarcastically ( to say someone silly). For example, if someone is deceived, we use 天真or单纯 to say he or she is too silly.

Well if someone say you are either 天真 or 单纯, it's not always a compliment. It can be a kind of satire to say you r like a kid. It means u should consider more than u thought. Well the meaning of these two words are similar. Usually the kids are 天真, because kids don't really think too much. They just let nature take it s course. 单纯 is to deacribe a person who have simple mind? Everybody can be 天真，but not everybody can be 单纯。