Learn French: Si l'extraterrestre visitait notre système solaire, il serait surpris. - If the alien visited our solar system, he would be surprised.

This sentence is a second conditional, describing a hypothetical situation. In French, similar to English (If the alien visited...), we use the imperfect tense (l'imparfait) after si to show the action is unlikely or imaginary, followed by the conditional (serait) in the second half of the sentence.

Why isn't si l'extraterrestre shortened with an apostrophe, like s'il?

The word si only drops its -i and uses an apostrophe when it is directly followed by il or ils (making s'il or s'ils). Before any other vowel, including the l' in l'extraterrestre, the -i remains intact.

Does surpris have an -s at the end because it is plural?

No, surpris is singular here, referring to one alien (il). The base form of the adjective (and past participle) surpris naturally ends in an -s even in the masculine singular. If the alien were female (elle), it would become surprise.

Is système feminine since it ends in -e?