Quote Originally Posted by impulse View Post
But why the conjunction "который" is mandatory and it cannot be just skipped as I did. If I skip it the sentence makes no sense or sounds defective?
Она сестра маленького мальчика, которого я видел вчера вечером в парке.

"Который" is the only way to join two clauses here.

English allows "fusion sentences" (I am not sure if it is a grammar term at all). Something like "I read the book you gave me". Here, "the book" functions both as an object of the first clause (I read the book) and as the object as the second clause (you gave me the book).

Russian just does not have this construction at all. The only way to join clauses is to use a conjunction word:
Я читал книгу, которую ты дал мне. (I read the book which you gave me).

If you say something like "Я читал книгу ты дал мне" it can be very confusing. The speaker will not probably get they are linked. It will sound as two independent sentences, the second incomplete: "1) Я читал книгу. 2) Ты дал мне ... (what? something is missing)" That's what the speaker would think. So, it is just not an option for Russian.

Quote Originally Posted by impulse View Post
And in the sentence you reformulated, you use "что та" which I dont know about. What is the meaning of it?
Моя девушка сказала, что та маленькая девочка - дочь её подруги Анны.

No, "что" and "та" are just two independent words

My girlfriend told me that (= что) that (= та) little girl is a daughter of her friend Ann.

Actually, "та маленькая девочка" just means "that little girl" (that one you mentioned before)

The confusion can raise of the fact in English "that" is ambiguous. The same word (that) has to different fuctions: 1) "she told me that..." and 2) "that little girl".

In Russian, they are just two different pronouns: "что" (relative pronoun, used as a conjunction) and "та" (indicative pronoun, feminine).

Quote Originally Posted by impulse View Post
Yes your proposal in the 3rd note is correct I exactly tried to say that.
Она уехала на работу на собрание.

Oh, I think now I see what you tried to say using "один". But you have chosen a wrong case.

Well, if it is an unknown meeting, and you want to specify you do not know any details, there is another option for you:

Она уехала на работу на какое-то собрание.

"Какой-то" is exactly the word which means "A meeting", "SOME meeting", "I-DON'T-KNOW-WHAT meeting".

Note that "один" has a different connotation. If you say: "Она уехала на работу на одно собрание", then it means you DO know what meeting she has, but you think it is not important to specify details which you know.

Quote Originally Posted by impulse View Post
So do you think that it is a good idea to forget about the past form of verbal adverbs and just think about the imperfective/perfective aspects?
Yes! According to the academic Russian grammar, the verbal adverb does not have any tenses. It only can be imperfective or perfective.

For example, there is a pair of verbs: читать (imperfective) - прочитать (perfective).

The first one is used to make the imperfective verbal adverb (читать - читая):
Я ужинаю, читая книгу. - I am having dinner while reading a book.
Я ужинал, читая книгу. - I was having dinner while reading a book.
Я буду ужинать, читая книгу. - I will be having dinner while reading a book.

Note that the tense does not matter (present, past or future). Only the simultaneous actions matter (both ongoing actions are performed at the same time).

The second one is used to make the perfective verbal adverb (прочитать - прочитав):
Я ужинаю, прочитав книгу. - I am having dinner after reading a book.
Я ужинал, прочитав книгу. - I was having dinner after reading a book.
Я буду ужинать, прочитав книгу. - I will be having dinner after reading a book.

Note that the tense does not matter again (present, past or future). Only the consecutive actions matter (one action is completed before another one is started).

Comment. "Прочитав" assumes the WHOLE book is read. If you only spent a short time reading a part of the book, then you use another perfective verb: почитать (infinitive) - почитав (verbal adverb).

Note that both "читав" or "прочитая/почитая" forms do not exist according to the academic grammar.

Although, to be honest, "читав" CAN be heard sometimes as a colloquialism: "Я ужинал, читав книгу". So, the speaker thinks that using "читав" specifies the action was in the past. But it is not considered as a part of the standard Russian, so do not bother learning it. It is already clear from the main verb (ужинал) that the action was in the past.
The opposite versions (прочитая/почитая) are just impossible under any circumstances.