Just a summary:
If you want to direct actions towards the actor (or the subject) of a sentence, you use si.
You add si to the end of the word and that’s all nice and good. For example, try applying si to I am making (I’m doing) to make it into I’m doing myself into... or I’m becoming.
1 translation: I am becoming good.
Same would be for the informal you:
2 translation: You (informal) are becoming bad.
Now, there are a couple of other rules of adding si to words which are pretty straightforward.
For all words you add si to except for aš, tu and jis/ji/jie forms (the singular forms), si at the end of the word gets shortened to simply s.
There is no magic about it: this i drops just because it’s easier to say it without i. It’s the same situation like jis matysi becoming jis matys in the future.
So, if you wanted to add si to daryti, you would get:
3 translation: To be becoming
That’s all good and nice. Try another one, first plain form:
4 translation: To be speaking
Add si to kalbėti to direct the action of speaking to who is doing it and mean to be speaking to oneself.
5 translation: To be speaking to oneself
It’s not an aš, tu or jis form so you only get s instead of si.
How would you say simply we are speaking:
6 translation: We are speaking.
Now, mes is also not a singular (aš, tu or jis) form so it also gets only s instead of si. How would you say:
7 translation: We are speaking to ourselves.
You have probably guessed kalbames and that should have been right. In speech, it’s totally cool. In writing, both mes and tes endings become mės and tės by changing that e to ė: it happens all the time and for all present, past and future or whatever forms.
So, mes kalbamės actually mean we are speaking to ourselves or we are speaking to each other or more commonly simply we are having a conversation.
How would you say:
8 translation: You (formal) are becoming good.
In this all, we have learnt one rule of si usage. There are two more rules to go. These rules, are, again, nit-picking and don’t worry too much about remembering them.
In the tu form, si does not want to touch the ending for tu which is i, so when they meet, si puts a separator e between itself and i UNLESS there is a before i which does not let the separator in.
This makes more sense if you look at an example. For example, tu kalbi + si would become kalbisi but si does not like i as an ending for tu so it puts e in the middle and it becomes kalbiesi.Kalbiesi could mean you are talking to yourself or you are talking yourself, talking what you have to say], having a conversation with somebody.
Another example is tu darai. It would become daraiesi but a does not let the separator in (there would be too little room for it) so it stays daraisi.
So, if jis prausia galvą was he is washing his head, how would you say, informally, you are washing your head:
9 translation: You are washing your head.
Look at how much easier it is to say prausiesi than prausisi - that e fits in there well.
So, here is another rule about the usage of si:
In the aš form, si does not want to touch the ending for aš which is u, so when they meet, si puts a separator o between itself and u UNLESS there is a before i which does not let the separator in.
The same thing, just a different separator o. It’s o because o sounds nicer. However much you wanted, you couldn’t say kalbuesi (well, you could but for some reason Lithuanians thought it wasn’t nice... I think it’s awesome, though).
How would you say:
10 translation: I am having a conversation.
11 translation: I am washing my hand.
That’s it for now.