I can speak Japanese.

dekiru: can, be able to
hanasu: speak, talk
nihon: Japan
go: language
nihon-go: Japanese

Watashi-wa nihon-go o hanasemasu.; I , Japanese ,can speak.: I can speak Japanese.

"can+verb" should be expressed by conjugating the verb in Japanese.
Or by changing the verb to a noun , we can say the phrase " someone can do..."

hanasu+koto: speaking(noun)
ga: "ga" will be added to an objective word.In some sense, "ga" is same with"o" after an objective word.

Watashi-wa nihon-go o hanasukoto ga dekiru.:I ,Japanese, speaking, can.
Watashi-wa nihon-go o hanasukoto o dekiru.: Japanese people understand this sentense, but "o"had not better use twice in one sentense.

In colloquial, "hanaseru" is much more used than "hanasukoto ga dekiru".

yomu: read
yomeru: can read

Anata wa hon wo yomeru. : you, book , can read,: You can read a book.

kaku: write
kakeru: can write

Kare-wa nihon-go o kakeru.: he,Japanese,can write: He can write Japanese.

iku: go
ikeru: can go
eki: station
ni: to

Watashi-wa eki ni ikeru.: I,station,can go, :I can go to the station.