Transforming (conjugating) verbs

Question

So I am studying from an online site called Memrise but they seem to teach pretty straightforward stuff that you have to memorize in order to do them. Yeah I can ask and say things but I can notice that the verbs change when the context changes. Some examples:

The verb is usually

はたらきます(to work)

but in this context

かれはオフィスではたらいています。(He works in an office)

it transformed to a brand new version of the verb. Now I have memorized them both but if I wanted to say something else I'm drawing blanks.

And here's another example. I noticed a pattern between these next few but I'm still not sure how they transform:

どうしてそんなにかなしんでいるのですか？(Why are you so sad?)

correct me if I bolded too much. I'm not sure if the の particle is included in the verb here. The verb is usually かなしんでいます but here it transformed to the bolded part. This one totally confuses me. It's different from the next one because in addition to having the verb have いる at the end it also has the の particle. I know it's a possession particle but its use here is beyond me.

And here's the third example where it's really similar to what we saw in the second example:

わたしはおなかがすいているから、おこっています。(I'm hungry therefore I'm angry)

Right here I can tell that the verb is the bolded part and the から particle is used to show a reason meaning therefore in this context.

Do these transformations have a name? Something I can research online or from some text book. I see that the verbs have these forms but I can't group them in one category so I can research them. They seem quite random.

There are many others that baffle me like "ふん" -> "ぷん" in some sentences. But I guess that's it for now. I really want to go a bit deeper in the grammar so I can improvise in my free time by creating sentences in my mind and not mess them completely up.

Answer 1

This is usually known as verb conjugation. Japanese verbs have a stem, and are then conjugated to change their meaning. In your first example, the stem is 「働{はたら}」, meaning to work. The second part is the conjugation, 「きます」, which is added on to the end of the verb stem to transform it into the "present indicative polite" form. You can see some of the conjugations of 「働{はたら}」 here.

Answer 2

彼｛かれ｝はオフィスで働いています｛はたらいています｝。

In this sentence, the bolded part splits into two pieces:

1. 「働いて｛はたらいて｝」(て-form of the verb　働く｛はたらく｝)
2. 「います」(ます-form of the verb いる)

The て-form of a verb has many uses, and in this case can be thought of as applying a progressive aspect to the verb 働く (similar to "-ing" in English).

The grammar construction of 「て-form」＋「いる・います」 constructs what in English would be a non-past progressive form of a verb. Thus, this sentence transaltesto "he is working in an office."

The rules regarding how to form the て-form are fairly regular but are a bit tricky to master at first. I'd recommend looking here for conjugation info as well as some examples of how this verb form is used.

どうしてそんなに悲しんで｛かなしんで｝いるのですか？

In this sentence, the bolded part splits into three pieces:

1. 「悲しんで｛かなしんで｝」(て-form of the verb 悲しむ｛かなしむ｝)
2. 「いる」(note: we do not use ます form because ます only can be used with the last verb in the sentence, if it is used at all)
3. 「の」

By the reasoning above, 悲しんでいる must mean something along the lines "is being sad" or "to have recently become sad." The の has a meaning of "thing" or "event," so 悲しんでいるのです roughly means something along the lines of "it is that you are sad." So, the entire sentence translates to "Why is it that you are so sad?" Using いるのです instead of います after the て-form gives the sentence a much lighter and caring feel.

私｛わたし｝はお腹｛おなか｝が空いて｛すいて｝いるから、怒って｛おこって｝います。

We have two pieces here again.

1. 「お腹｛おなか｝が空いて｛すいて｝」(て-form of the verb お腹｛なか｝が空｛す｝く, to be hungry)
2. 「いる」

I'm guessing at this point, based on the prior two examples, the reasoning behind this construction and what it's trying to convey should now be clearer. :)