常(常) cháng(cháng) means “often” and is almost always interchangeable with 經常经常 jīngcháng. It can sometimes imply a slightly more casual or less frequent occurrence than 經常经常 jīngcháng. To negate it, you must drop one 常 cháng, becoming 不常 bùcháng.

Using 常 cháng by itself with a simple verb also indicates a future frequent action in spoken Chinese — a casual, friendly suggestion.

經常经常 jīngcháng implies a regular or frequent occurrence, and can be used in both positive and negative contexts. If there’s a subject, it always comes after it. When used as an adverb, it’s interchangeable with 常常 chángcháng — but it can also act as an adjective before 的 de + noun, a role 常常 can’t fill.

Note: 經常经常 jīngcháng and 時常时常 shícháng work almost the same way, but 經常经常 jīngcháng implies a higher frequency or regularity than 時常时常 shícháng.

往往 wǎngwǎng means “often” in the sense of “frequently” or “typically.” Unlike 常常 chángcháng, which describes a habit or routine, 往往 wǎngwǎng implies a tendency and describes a pattern or trend based on past experience — a conclusion or judgment about a regular situation.

You can always use 常常 chángcháng to replace 往往 wǎngwǎng, but not the other way around, since 往往 wǎngwǎng always needs a certain condition or situation behind it. It also has no negated form of its own — negate the verb that follows it instead.

平常 píngcháng refers to the normal or ordinary state of things — a regular, typical situation that isn’t particularly noteworthy. When used as an adverb, it’s interchangeable with 常常 chángcháng; it can also work as an adjective before 的 de + noun, which 常常 can’t do.

Like 往往, 平常 píngcháng has no negated form of its own — negate the verb that follows it instead.

時常时常 shícháng works almost exactly like 經常经常 jīngcháng — it’s an adverb that goes after the subject, and can be used in both positive and negative contexts. The main difference is nuance: 經常经常 jīngcháng implies a higher frequency or regularity, while 時常时常 shícháng suggests something happens a bit less often or less predictably.