A customary saying in Essos and a common greeting in Braavos, "valar morghūlis" is traditionally answered with "valar dohaeris".

Also the following are my own notes:

• "Valar" is in the collective plurality (vs. singular, plural, etc.), in the nominative case.
• "Morghūlis" and "dohaeris" are in the third-person singular aorist tense.

The aorist tense refers to an action based on the past but without qualified reference to its place in time, duration, or completion ("Men die [at unspecified times in the past, present, and/or future]"). Different from ("Men die [right now]"), ("Men died [in the past]"), ("Men died [in the past, continuously/habitually]"), etc.