常用

常用 answers 'is this thing widely used?' So it parks next to a thing: 常用词, 常用方法, 常用工具. 常 and 常常 answer 'how often do you do this?' So they park next to an action: 我常去那家店, 我常常加班. People mix them because English 'often' covers both. But 我常用北京话 is not a frequency word at all — it's 'I often use Beijing dialect', and 北京话 is the object of 用. If you want to talk about your routine, drop 用 entirely and say 我常去 / 我常常去. That 用 is the whole difference: keep it and you owe the sentence a thing being used, drop it and you're free to talk about habits.

常用说 looks tidy in English but leaves 用 with nothing to use. Chinese 用 never floats alone; whatever you're using sits immediately after it. So the slot order is fixed: 常 + 用 + [the thing] + [what you do with it]. 常用北京话说 means 'often use Beijing dialect to say'. Same skeleton in 常用手机付钱 and 常用微信联系我. If you only want 'often say', don't borrow 用 at all — 常说 does the job. That's the whole test: is there a tool in the sentence? If yes, keep 常用 and put the tool right after it. If no, use 常说.

“他们常用北京话说：新鲜啊！” (They often use Beijing dialect to say: 'How novel!')

Learners see 常用词 and 常用方法, assume 常用 is glued to nouns, then panic when it stands alone. It doesn't have to. To comment on a word, an app, or a shortcut, say 很常用 — 这个功能很常用. Using 是 instead leaves the sentence hanging, like saying 'this word is commonly-used' with the 'used' missing. Flip it either way and it still works: 常用词 or 很常用的词. Just remember the negative side too, where 不太常用 is the natural choice and 不很常用 sounds off.