Felix diēs patris! is a Latin equivalent of the English phrase "Happy Father's Day!" The phrase translates literally as "Happy Day of (the) Father!" in English. The pronunciation will be "fey-leeks dee-eys pa-trees" in Church and classical Latin.
"He" is English is the personal pronoun is in Latin.
"About (concerning, of) fathers" is an English equivalent of the Latin phrase de patres. The pronunciation of the masculine plural prepositional phrase in the (therefore) ablative case will be "dey pa-treys" in the Latin of ancient Rome and of the Church.
The verb est in Latin is "is" in English.
Testamentum in Latin is "testament" or "will" in English.
"Short" in English is brevis in Latin.
"Mind" in English is mens in Latin.
Quī in Latin means "what" or "which" or "who" in English.
M in Latin is "1,000" in English.
Infernus in Latin is "hellish" in English.
"Family" in English is familia in Latin.
"Life" in English is vita in Latin.
"Humility" in English is humilitas in Latin.