Learn Arabic (Modern Standard): من سكت نجا.

While you probably first learned مَن as the question word "who?", it is also used to mean "whoever" or "anyone who" in statements. In this sentence, it sets up a general condition rather than asking a question.

Why are the verbs سكت and نجا in the past tense if the English translation says "keeps silent" and "survives"?

In Arabic, conditional sentences (like those starting with "if" or "whoever") and timeless proverbs very often use the past tense. Using the past tense verbs سكت (he was silent) and نجا (he survived) shows that the outcome is absolutely certain. In English, we usually express general facts like this using the present tense.

Why isn't there a word for "then" before نجا (survives)?

You don't always need a connecting word in Arabic! When using مَن to mean "whoever", you can simply place the condition verb (سكت) and the result verb (نجا) right after each other. The structure itself implies "whoever does X, Y happens."

How can I tell if the first word is مَن (whoever) and not مِن (from) since they are spelled the same?

They are spelled exactly the same way in Arabic when the short vowel marks are left out. You have to rely on context. If you read it as the preposition مِن (from), the sentence would mean "From kept silent survived," which doesn't make sense. The context tells you it must be the pronoun مَن (whoever).