Learn Arabic (Modern Standard): لم نخف من الانفجار المفاجئ.

The word الانفجار (the explosion) is the verbal noun of the Form VII verb انفجر (to explode). Form VII verbal nouns follow the pattern انفعال (infi'aal), adding 'in-' to the beginning of the root and a long 'aa' before the last root letter. With the root ف ج ر, we get انفجار (infijaar).

Why is the verb written as نخف instead of نخاف?

The negative word لم (did not) forces the present-tense verb into the jussive state. The normal 'we' form is نخاف (nakhaaf). However, in the jussive, the final letter gets a sukoon (no vowel). To avoid having two unvowelled letters in a row (the long 'aa' and the 'f'), Arabic drops the long vowel, shortening نخاف to نخف (nakhaf).

Why do we use the word من (from) here when the English translation just says 'fear the explosion'?

In Arabic, the verb خاف (to fear or be afraid) is almost always followed by the preposition من (from or of) to indicate what you are afraid of. Literally, the Arabic sentence says 'We did not fear from the sudden explosion.'

Why does the adjective المفاجئ (the sudden) end with the hamza on a 'yaa' shape (ئ)?