'Once in a while' (two words) is correct.
It means once in every two years.
Triannually, but it is seldom used in conversation.
Twenty years is two decades.
Once a year or Once every two years
after it shakes once say ''one'' after the second say ''two'' after the third shake say ''three''
Car transmissions should be flushed at least once every two years, that's what most car manufactures say and mechanics
In Spanish, you don't normally say "I am so many years old", but rather "I have so many years." In this case, it would be "Tengo once años". You can also say "I am 11 years of age", or "Soy de once años de edad."
Wombats may reproduce once a year, or once every two years.
it means the do two things at once
yes
Once every two years