To make the past form of a
る-Verb in Japanese, just like all other conjugations of
る-Verbs, you will need to remove the る first. After this, all you need to do is add the auxiliary verb た (casual), or ました (polite).
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ドアを開（あ）ける。
I open the door
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ドアを開（あ）けた。
I opened the door.
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ドアを開（あ）けました。
I opened the door.
ました is actually just ます conjugated with た on the end of it, so it is 2 auxiliary verbs, not just one. This means that only た gives the meaning of ‘past’ to a word, while ます keeps it’s regular role of making the verb more polite.
When た is added to verbs (or any other word), it can have one of three possible meanings.
過去（かこ） - Happened at some point in the past. (Did/Was)
完了（かんりょう） - An action that has finished ‘happening’. (Done)
存続（そんぞく） - A new state of something that is continuing. (Has been done)
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たかしは1994年（ねん）に生（う）まれた。
Takashi was born in 1994. (An example of 過去（かこ）)
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今朝（けさ）、テストを受（う）けた。
I took the test this morning. (An example of 完了（かんりょう）)
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彼（かれ）の名前（なまえ）を忘（わす）れた。
I forgot his name, and still can’t remember. (An example of 存続（そんぞく）)