他提出了一个新的理论来解释这种现象。

Tā tíchūle yī gè xīn de lǐlùn lái jiěshì zhè zhǒng xiànxiàng.

He proposed a new theory to explain this phenomenon.

理论上这个方案是可行的，但实际操作起来很难。

Lǐlùn shàng zhège fāng'àn shì kěxíng de, dàn shìjì cāozuò qǐlái hěn nán.

In theory, this plan is feasible, but it's very difficult to implement in practice.

学习理论知识固然重要，实践经验也不可少。

Xuéxí lǐlùn zhīshi gùrán zhòngyào, shíjiàn jīngyàn yě bùkě shǎo.

Learning theoretical knowledge is certainly important, but practical experience is also indispensable.