in reply.

The indefinite past tense is formed by adding the word 了 _liao_ or _lo_
"finished":—

他来了 _t'a lai lo_ = "he come finish," = "he has come."

This may be turned into the definite past tense by inserting some
indication of time; _e.g._

他早上来了 = "he came this morning."

Here we see that the same words may be indefinite or definite according
to circumstances.

It is perhaps more startling to find that the same words may be both
active and passive.

Thus, 丢 _tiu_ is the root-idea of "loss," "to lose," and 了 puts it into
the past tense.

Now 我丢了 means, and can only mean, "I have lost"—something understood,
or to be expressed. Strike out 我 and substitute 書 "a book." No Chinaman
would think that the new sentence meant "The book has lost"—something
understood, or to be expressed, as for instance its cover; but he would
grasp at once the real sense, "The book is or has been lost."

In the case of such, a phrase as "The book has lost" its cover, quite a
different word would be used for "lost."