I can do that.

...dekiru : can... , be able to ....

sore, are : that
suru : do
dekiru : can
(conjugation : dekinai,dekimasu,dekiru,dekirutoki,dekireba,dekiro)

Watashi wa sore o dekimasu : I, that, can : I can do that.

hanasu : speak
nihongo : Japanese
-eru : can..., be able to...
-emasu : (polite)can..., beable to...
hanasemasu : hanasu+-emasu : can speak

Watashi wa nihongo o hanasemasu. : I, Japanese, can speak : I can speak Japanese.

if "-eru" or "-reru" is attached to a verb, it means " can...." with verb conjugation.

More easier way to mean " can..." is ;
1. put "-koto" behind the verb(plain) , which makes the verb a noun.
2.put " -ga dekimasu" after the word.

For example,

Watashi wa nihongo o hanasu koto ga dekimasu (hanasemasu).; I, Japanese, speaking, can : I can speak Japanese.

yomu : read
(conjugation : yomanai,yomimasu,yomu,yomutoki,yomeba,yome)

Watashi wa hon o yomu koto ga dekimasu (yomemasu). : I, book, reading, can, : I can read a book.

Conjugaiton of verbs is difficult so you can use this easy way to make "can do " sentense.