English uses the same word in "I open the door" and "the door opens". Japanese usually uses two different verbs, and choosing the wrong one changes which particle you need.
A paired example
doa wo akeru, I open the door (transitive, takes を)
doa ga aku, the door opens (intransitive, takes が)
開ける (akeru) means someone opens something. 開く (aku) means something opens by itself or simply is in the state of opening. The transitive verb takes a direct object marked with を. The intransitive one takes が and has no object.
More common pairs
hajimeru, to start something / hajimaru, something starts
shimeru, to close something / shimaru, something closes
tsukeru, to turn on / tsuku, to come on
Why it matters
The pair you choose shapes the whole sentence. Use the transitive verb and the listener expects an actor and an object. Use the intransitive one and the event just happens, with no one blamed or credited. Japanese leans on intransitive verbs to describe results without pointing a finger, which is part of why translated-from-Japanese English can sound oddly passive.
Learning them
There is no perfect rule for guessing which form is which from spelling, so most learners memorise the pairs together. Seeing them side by side, as above, makes the pattern stick faster than meeting them one at a time.
Test one of these examples in the translator and see how it comes out.
Open the Translator