Setting impression criteria
On Yandex.Search, ads may be served to users whose search queries contain the keywords that you specified in their entirety. For example, if you set the keyword tours to outerspace for your banner group, your banners will be triggered by the search queries tours to outerspace, discount tours to outerspace, etc., but they will not be triggered by the queries tours or outerspace.
When displaying ads, the system accounts for various forms of a word within the same part of speech, including singular and plural forms. For example, if your keyword is tours to mars, your banner will display if the user searches for tour to mars or tourS to mars.
Read more about keywords.