Learn Polish: Szpieg chciał przechwycić naszą wiadomość. - The spy wanted to intercept our message.

Nie sposób is strictly impersonal, so you must drop the subject (szpieg). To make it past tense, add było, giving us: Nie sposób było przechwycić naszej wiadomości. Notice that because nie sposób is an inherently negative expression, it triggers the genitive case for the direct object, changing naszą wiadomość (accusative) to naszej wiadomości (genitive).

How does the prefix 'prze-' change the base verb 'chwycić' to mean 'intercept'?

The base verb chwycić means 'to grab', 'to seize', or 'to catch'. The prefix prze- frequently indicates movement across, through, or over a space. By combining them into przechwycić, the literal imagery becomes grabbing something while it is traveling across from one point to another—which is exactly what it means to 'intercept' a message.

Could I use the imperfective form 'przechwytywać' instead of 'przechwycić' here?

You would generally only use the imperfective przechwytywać if the spy wanted to engage in the ongoing, continuous process of intercepting messages over time. Because his goal here is the successful, one-time completion of grabbing this specific message, the perfective przechwycić is required.

Since 'wiadomość' ends in a consonant, how do we know it requires the feminine pronoun 'naszą'?