As Jessie says, it is indeed easier to understand when thinking in terms of pairs, in this case, by contrast with -ít. I would like to add to what she wrote that with the verbs in -ul/-ül the action is sort of made by the subject on itself, wherease with -ít the same action is performed by someone else. Let's see with two examples: (A) tanulni vs tanítani, and (B) mozdulni vs mozdítani. In (A), the 'action' is about 'bringing knowledge'. If a person does it on themselves, that is to say, 'learns' then we use the verb 'tanulni'. If a person bring the knowledge onto another one, that is to say, teach them, then we use tanítani. In (B), the 'action' is about 'moving'. Funnily enough, we use the same word in English ('The table moves', something you can say during a spiritism session, or 'I moved the table'). In Hungarian we make the difference (you can guess which one is which).

Csoportosul belongs to the first case.

Lets separate it into two different things: ul/ül can make verb from an adjective 'A' and the resulting verb will mean: to become 'A' some examples: szép - szépül -> to become nicer sárga - sárgul -> to become yellow (sárgul az irigységtől) buta - butul -> to become dumb However, ul/ül can be used only a few cases like this. -odik/-ödik seems more frequent to me and it works exactly the same way. világos - világosodik -> to become lighter (Hajnalig fenn voltam. Már világosodott, amikor lefeküdtem.) nagyobb - nagyobbodik -> to become bigger rosszabb - rosszabbodik -> to become wronger (A beteg állapota rosszabbodott és azonnal meg kellett műteni) piros - pirosodik -> to become red (Nemsokára érik a paradicsom. Már kezd pirosodni.) =============== Tanul, mozdul, fésül, örül, etc. are very different thigs. First, there are NOT such words as tan, mozd, fés, ör These are all separate, independent verbs. It does not worth to think too much about their endings just learn them as they are. By the way, ul/ül express some kind of reflexivity in these cases.

I think Jessisamess gave a smart explanation, just a short correction. Sorry about that Jess :))) We don't use 'A nap barnít az embert' but instead 'A nap barnítja az embert'. The sentence rather implies to the effect of the sun rays on the human skin. So I would translate this sentence as: The sunbathing makes the man's skin browner.

Köszönöm szépen! It helps a lot! Another mistery revealed. :D