胆大心细的英文：
be bold but cautious

参考例句：

He is bold but cautious and he always keeps calm when anything comes up.
他这人胆大心细，遇事不慌。

bold是什么意思：
adj. 大胆的，无畏的；莽撞的；黑体的；厚颜无耻的；险峻的；醒目的

She is a bold piece.
她是一个放荡的女人。

The Government resolved on a bold policy.
政府决定采取大胆的政策。

This is bold and exuberant cooking.
这是大胆而又精彩的烹饪。

This is a bold-faced lie.
这是个厚颜无耻的谎言。

A cook is Bold on his own dunghill
狗仗人势

cautious是什么意思：
adj. 谨慎的，小心的

Be cautious of thieves!
提防扒手！

A cautious mind is the half of wisdom
谨慎小心近于明智

He is cautious of telling secrets.
他留心不泄密。

Losses make us more cautious.
损失使人更谨慎。

In dealing with the problem he was cautious to an extreme.
他在处理这个问题时谨慎得过分了。