This is a tough question! For me, as a native speaker of American English, there is only a small difference in my daily use of these words. I tend to use 'constantly' to express a habitual action that is annoying or unpleasant. Examples: "His young brother constantly makes fun of him (e.g. ridicules him)." or "The teacher is constantly pointing out her mistakes." A more colloquial alternative to 'constantly' is 'always.' "His young brother always makes fun of him." or "The teacher is always pointing out her mistake." Again though, that is really only for colloquial speech since it's an exaggeration. 'Frequently,' to me, feels more formal/professional. Examples: "He frequently cites a particular author when he discusses this issue." "We've frequently discussed the possibility of opening a new office there." 'Regularly' can often be used as a synonym; however, you should definitely use 'regularly' if the action you are describing is repeated with a specific time-frame... i.e. "we regularly start staff meetings at 4:00, but today we started at 4:30." "We've gone to grandma's house pretty regularly since we were children." 'Seldom' is a funny word. Many native speakers will actually say 'seldomly' because they are trying to make it a regular adverb and don't realize they are making a mistake. 'Seldom' is sometimes a perfect synonym with 'rarely,' so that, "We seldom/rarely go to grandma's house anymore." 'Seldom' seems to me to be slightly more formal than other similar adverbs. In colloquial speech, we'll often say 'hardly ever,' "We hardly ever go to grandma's house anymore," or "They hardly ever see each other these days." Hope that helps!