It seems 到, 到了, 了 can all be used to indicate completed actions.
What's the difference in terms of indicating an action has finished?
For instance, this grammar page offers a number of examples:
Nǐ xiǎngdào le shénme hǎo bànfǎ?
Have you thought of any good methods yet?
Wǒmen jīntiān xué le hěn duō shēngcí.
Today we learned many new words.
In the first example, does the meaning change if you say 想到 instead of 想到了? What if you say 想了?
In the second example, does the meaning change if you say 学到了 instead of 学了? How about 学到?