常、经常

常 is a real word, not a chopped-down version of 经常 waiting for a second half. Say 我常去北京 and no one blinks. The difference is weight, not correctness: 经常 is the spoken everyday default, while 常 sounds tighter and snappier. There is also 常常, which is basically 常 with a little bounce — 我常常去那儿. Match the word to the rhythm: a long relaxed sentence takes 经常, a short punchy or written line takes 常. Example in the wild: “他们常用北京话说：新鲜啊！” ── 《这还是茶馆儿吗？》

English 'often' hops around freely, but Chinese locks the word into one slot. The order is subject, then 常 or 经常, then the verb doing the action. So 我经常坐地铁上班 is the shape you want, every time. Slide it after the verb — 我坐经常地铁 — and it sounds like a broken robot line. Push it in front of the subject — 经常我坐地铁 — and it sounds like a headline fragment, not a sentence. The same slot holds other frequency words like 有时候, so learn the position once and reuse it.

Both 我不常去 and 我不经常去 are correct Chinese, so stop worrying about that choice. 不常 is short and offhand, while 不经常 spells the idea out and lands with more weight. Reach for 不经常 when you are pushing back on someone: no, not that often. The real trap is 没, which handles things that did not happen or that you do not have. It has nothing to do with how often something happens, so 没常 and 没经常 are simply broken. See 不 vs 没 for the full split.