You can remember the meaning of 天真 with this easy mnemonic:
You are so <strong>naive</strong>, do you actually think that the <strong>sky</strong> (天) will fall on you if you are naughty? That is not <strong>true</strong> (真) at all!
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.