It doesn’t translate directly - That helps me a lot.
The focus of this lesson is "How to use 'help'." Since "help" and the Japanese word tetsudau (手伝う) are not perfect equivalents, the title "That helps me a lot" cannot be translated as Sore wa watashi o tetsudatta.
The word tetsudau literally means to "lend a hand" to another person. More specifically, it implies working together with someone to assist them in what they are doing. Therefore, it is used in situations like these:
Tomodachi wa watashi no shukudai o tetsudatte kureta. (My friend helped me with my homework.)
Tetsudaimashou ka? (Shall I help/lend a hand?)
Now, let’s look at the following English example:
Your advice helped me.
If you use tetsudau here, it results in a very strange Japanese sentence: "Anata no adobaisu wa watashi o tetsudatte kureta" (??). While giving advice is an act of "helping" or "assisting," you aren't physically "working together" with the person. In Japanese, tetsudau is reserved for practical actions, such as helping with homework or carrying heavy luggage.
Let's look at more examples of "help" where tetsudau is not used:
This medicine will help your pain. : kono kusuri wa itami ni kiku。(kiku: to be effective, to works for)
My English helped me while I was traveling.: watashi no eigoryoku wa kaigai rykou chuu ni yaku ni tatta。(yaku ni tatsu: to be useful/helpful)
Every little bit helps.: donna chiisai koto demo yaku ni tatsu。(yaku ni tatsu: to be useful, to contribute)
This way helps clean up the ocean.: kono houhou wa umi o kirei ni suru koto ni yaku ni tatteiru。(yaku ni tatsu: to contribute to)
It seems that in many cases, yaku ni tatsu (to be useful/helpful, to contribute to) is a better fit than tetsudau. However, you have to be careful; for medicine, kiku (to be effective) is more natural than yaku ni tatsu. You can't just use one Japanese word for everything.
Going back to the start, if we translate "That helps me a lot," sentences like these would be much better:
Sore wa sugoku yaku ni tatta. (That was very useful.)
Sore no okage de, tasukatta. (Thanks to that, I was saved/helped out.)
Words like "help," "have," and "work" have broad meanings in English and often cannot be translated directly. To master them, you need to categorize their specific English usages and think about the most natural Japanese equivalent for each context.