MyBB Community Forums

let's say we use all forums for search. the search result display will show all the forums, categories. now in search result display page is there anyway way can filter the category, forum?

In default theme, search results have an identifier for the column at the top. If you click on the header "Forum" the results will be sorted by that column. Goes for Post, Author, Forum, Replies, Views and Date Posted. You can tell which column is sorted by asc or desc after the column name. For ascending or descending. Change the sort order from one to the other by clicking on it.

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Your results will be grouped by forum, but the filter is pre-applied in the search setup.

Yes, doing a search and filtering by forum doesn't make a lot of sense - the forums you want to filter by are selected at the start.