While 置（お）きに may appear to be a 副詞（ふくし） (adverb), it is actually a combination of 置（お）き, a 接尾後（せつびご） (suffix) meaning ‘opening’, and the 格助詞（かくじょし） (case marking particle) に. 置（お）きに (often without kanji) is an expression used to show that something happens ‘every (A)’, or ‘at intervals of (A)’.
is technically a suffix, it will be attached to the end of numbers or counters that it is highlighting as being an ‘opening’.
Please drink this medicine every other day. (Every second day)
I was told by the doctor to come to the hospital every other month. (Every second month)
I can't relax if I don't clean my room every two days.
It is very common to get ごとに
, and おきに
mixed up. While ごとに
has the literal meaning of ’every’, おきに
has the literal meaning of ‘opening’. Due to this, おきに
will not always mean ‘every’. Let’s look at some examples of when these two expressions are the same, and when they are not.
Discrete variables (things that are usually measured upon completion) - Only ごとに
may be used to express ‘every’. For example, a day, a week, or a month may easily be divided, but they are usually counted as whole units. Due to this, ごとに
will mean ‘every week’, while おきに
would actually mean ‘every second week’ (because there is an opening of one week).
I use my eye drops every other day.
Every other time I go out, I forget something.
Continuous variables (things that are usually measured in fractions) - Either ごとに
may be used. For example, seconds, minutes, and hours are usually measured by fractions, especially considering sports and racing. In this case ごとに
will mean ‘every second’, while おきに
will mean ‘every opening of one second’ (disregarding all of the milliseconds, this means every second).
I move my body every 2 hours, everyday.
I move my body every other 2 hours, everyday.
This is a topic that native speakers frequently get wrong themselves, so don’t worry if it is a bit tricky! If you are unsure about which one to use, think of 置（お）きに as ‘every other’ for most pursposes.