Learn Spanish (Latin America): Me sorprende lo arriesgado que es saltar con la tabla. - It surprises me how risky it is to jump with the board.

In Spanish, to express "how [adjective]" inside a larger sentence (as a degree or intensity clause), you use the formula lo + adjective + que. You can use ¡Qué arriesgado! as a standalone exclamation ("How risky!"), but you must use lo arriesgado que when connecting it to a main verb like sorprende. Note that you cannot use cómo directly with an adjective.

Since Me sorprende expresses an emotion, shouldn't the verb es be in the subjunctive (sea)?

This is a common tricky spot! Normally, verbs of emotion trigger the subjunctive, like in Me sorprende que saltar sea tan arriesgado. However, the structure lo + adjective + que focuses heavily on the extreme degree or intensity of something already experienced and accepted as a solid fact. Because of this focus on the absolute reality of the degree, it usually takes the indicative (es).

What is the subject of the verb sorprende here?

The subject is the entire chunk of information: lo arriesgado que es saltar con la tabla. The verb sorprender works exactly like gustar. The literal translation is "How risky it is... surprises me." Since the subject is a singular clause, the verb is conjugated in the third-person singular.