Ah this one is easy ... ^o^
Good Morning – subha udasanak - සුභ උදෑසනක්
Good Night – subha rathriyak - සුභ රාත්රියක්
In here subha (සුභ) means good , auspicious or nice
So using the word subha we can construct many other sentences.
Ex: subha gaman - සුභ ගමන් - have a great journey
But the word subah (සුභ ) is too formal to be used elsewhere.
Usually Sinhalese use the word Honda (හොඳ ) when they want to say that something is good.
Ex:
Honda panak - හොඳ පෑනක්- Good pen
Honda pothak - හොඳ පොතක් - Good book