Reflexive verbs. I'm still puzzled
I'm still puzzled when it comes to reflexive/pronominal verbs. According to my understanding, reflexive verbs are used when the action is done to oneself. In other words, the subject and the object are the same. I understand this part, but this rule can be followed/understood when it comes to action verbs. For example, wash, shave, kill, hit etc. But what about non-action verbs. Like "olvidar", "llamar" and "acordar". Why does one say, "no te olvides comprar el regalo." not "no olvides comprar el regalo"?
2 Answers
If you keep thinking of reflexive verbs as verbs done to oneself (or confusing reflexive verbs with pronominal verbs, for that matter), you are making this harder for yourself.
First point of enlightenment which you can skip if you are not a grammar freak
A reflexive verb is a type of pronominal verb. Pronominal verbs accept a 'clitic' (informally an object pronoun such as 'me', 'se' etc.). Reflexive verbs, being a subset of pronominal verbs, similarly accept the clitic. A reflexive verb is a pronominal verb that obviously refers the action right back at the subject. The classic example is 'afeitarse'/'lavarse', to shave/bath oneself. When you bath, you do bath yourself, after all. When you shave, it's your beard that falls into the sink (hopefully).
There are more types of pronominal verbs, however, with the most infamous being the passive pronominal, which hints at an action being somewhat performed without an agent. La situación se va a convertir en un drama - The situation will turn into a drama. The situation is not 'converting itself' into anything (hell, it isn't even alive!).
Emphatic pronominals tend to make the effect of a verb stronger. A good example is comer. 'se lo comío' converts 'lo comió' from 'he ate it' to 'he ate it all up'.
And then there's intrinsically pronominal verbs which are just there to make your life miserable. Most of the time, the differences between the original form and reflexive form of the verb is a 'native's nuance', something you won't really understand too well. I think reírse might be a good example, since I haven't found any explanation of the verb in detail anywhere on the net, or how it differs from 'reir' (aside from being used a lot more in situations that have turned it into a 'laugh at' verb).
Second point of enlightenment which will help you solve your current problem
If you read the above (like an awesome nerd), you can figure out the answer to your question, but I'll explain it here.
Spanish 'reflexive' verbs (really, they are pronominal) often don't make intuitive sense. In many cases, there is no apparent need of reflexivity. In such cases, the 'reflexive'-ness is because of some other reason, such as making the verb more emphatic, creating a 'passive' look, making your life miserable etc.
Olvidarse is used for a passive look in case of objects. 'me olvidé de tu cumpleaños' does not carry the same connotation as 'olvidé tu cumpleaños'. The latter implies something like 'I left behind the memory of intentionally/I neglected to', which can sound rude. 'me olvidé de tu compleaños' sounds more like 'it slipped out of my mind'. This point is something I am not completely sure about, however (I have seen conflicting discussions on the net). For the most part, olvidar and olvidarse are used in the same way, so there's a distinction you will have to make based on where you are located (in some places, you can't use the olvidarse de construct).
A difference is with the passive construction using olvider. 'se me olvidaron las llaves' changes focus from you to the keys. 'The keys were forgotten (by me)". Often used to stress that you didn't actually forget the keys on purpose.
llamarse is, simply put, 'tener por nombre'. You might be tempted to think of 'me llamo' as "I call myself", but that's not the same thing. You can call yourself Badshash Darth Vader Akbar the Great King of the Sands of Arceus, but your 'me llamo' is still your name. Remember that the verb llamar can be used to mean 'someone is called something'. 'Me llaman loco' is 'they call me mad', plain and simple, but 'me llaman' uses the ordinary form of llamar, not llamarse or anything. "Me llamo a mi mismo" = I call myself !== "me llamo" = "My name is ".
Acordar changes its meaning with 'reflexive'-ness. The pronominal acordarse does not mean 'I remind myself' but 'I remember'. This is a case of meaning change from original to reflexive form. If 'I remind myself' helps you to remember what acordarse means, go ahead. But remember that acordarse has a separate meaning 'to remember', and when a native hears it, he is probably (I won't stress this too much, but if it really was that simple, dictionaries would list acordarse as 'acordarse ~ to remember/to remind oneself') not thinking of reminding himself of anything.
I don't know if I managed to help you, but all of this stuff should provide you with some pleasant reading while we wait for a native to (hopefully) chime in.
P.S:
Here's something from wordreference concerning the olvidar topic:
When talking about actions the meaning is pretty much the same: olvidé sacar la basura = me olvidé de sacar la basura (I forgot to take out the trash). On the other hand, when it comes to physical objects, there might be a slightly difference in meaning: olvidé las llaves (I left the keys) vs. me olvidé de las llaves (I forgot about the keys). The first sense can be reinforced by the use of the reflexive form: me olvidé las llaves (I left the keys behind)
As Helado pointed out you can get into a morass of ambiguity trying to explain the difference because verbs like olvidar and olvidarse can be virtually interchangeable.
Here is at least one distinction that seems straightforward. Possibly already stated.
When you say "olvidé mis llaves", it could mean that you left them behind or it could mean that you no longer think about them at all, that you have forgotten about them in a general sense.
When you say "me olvidé mis llaves", or "se me olvidó mis llaves", you clairify that you simply forgot to bring them with you, not that you have permanently forgot about them.
