命 (inochi) is more of an abstract or philosophical concept: the idea of the life force itself, or the quality of being alive.
人生 (jinsei) refers specifically to human life (as you might guess from the character 人 in the word) as it is lived from birth to death.
***
Bonus tip: When you’re not sure about differences in usage and nuance, it really helps to look at sample sentences. Here are some sample sentences with 命 and 人生 :
命：
… (more)
- １００万人の人々がその戦争で命を落とした。One million people lost their lives in the war.
- 虫歯を侮ると命に係わる場合もあるのです。Making light of cavities can be a matter of life and death.
- 身命をかけて職務を実践し、すべての警察官の心に火をと
Inochi is life in general: the state of being alive; your life; my life; a fish’s life.
Jinsei is, as you can tell by the kanji, a human’s life. I think of it almost as “lifespan”: it could refer to all of the things you might do in your lifetime.
If someone went to a great college, got a good job, got a wonderful spouse and beautiful children and was happy every day, you would say that he or she had a good jinsei. Or if someone struggled with poverty and alcoholism their whole life and died alone in a gutter, you might say that he or she had a bad jinsei. Sure, you could translate this jinsei as “life” as well; but it is “life” as in the span of time, as in the specific 80 years or so that that person spent on Earth, as opposed to the more general or philosophical.
生活 seikatsu refers to either everyday life (day to day living) or to one’s livelihood (getting by economically). If your life is tough, you use seikatsu. If you want to talk about the process of living (working, eating, sleeping, etc.), you also use seikatsu.
人生 jinsei refers to life viewed as a whole. If you talk about jinsei, you will talk about the things that happen to you during life and the stages in your life — a person’s life viewed as a totality. Probably suitable as a translation of “This is your life”.
命 inochi is life as the state of being alive. If you spare someone’s life, you use inochi. If you are fighting for your life, you use inochi.
生きること ikiru koto also means the state of being alive or surviving (not dying).
No need to make はvsが so complicated. It can be made pretty simple.
Let's take one particularly awesome example. An alien has transformed into an exact replica of me. You have the gun, and you don't know which one is which. Who do you shoot?
"I am Koichi" one of us says
"I am Koichi" the other one says
This, essentially, is the difference between は and が. let's translate these sentences into Japanese.
私はこういちです。 I am Koichi
私がこういちです。 I am Koichi
Shoots, the translation is the same, so what's the difference? Here's where people usually get confused. First, let's take a look at は and が, and what they mean.
は emphasizes what comes after it and talks about something
が emphasizes what comes before it and identifies something
So, what's the difference between these two sentences?
私はこういちです。I am Koichi. This sentence uses は, so the important thing is what comes after it. The important thing is that my name is Koichi (and not that I am the Koichi you're talking about).
私がこういちです。I am Koichi. This sentences uses the が particle, so the important thing is what comes before the が. The important thing here is that I am the Koichi you are talking about (remember, は talks about something). We are identifying something.
Let's look at another example.
I'm introducing myself. I would say:
私はこういちです。 I am Koichi. The important part is that my name is Koichi.
Now we're in a crowded room, and the announcer comes on and says "Someone named Koichi left their car lights on. Who here is Koichi?"
Then you'd reply: 私がこういちです, because you need to identify that you are the Koichi they're talking about.
When it comes down to it, though, it can be summed to these two points:
は emphasizes what comes after it and talks about something
が emphasizes what comes before it and identifies something
すしが好きです = I like Sushi
We're identifying the Sushi.
すしは好きです = I like Sushi / It is the sushi that I like
We've already identified the sushi, we're already talking about the sushi, so there's no need to identify it. Now we're just talking about the sushi, so は is the particle to use.
In the end, though, it's going to come down to experience. People make mistakes with は and が all the time, including Japanese people sometimes. It's a tough one, and the difference, while important, is also pretty nominal. Practice a lot, and make sure you study every single instance of は and が that you run into and figure out why は is used or why が is used. If you don't do this, you'll never learn (fluently) why they're different and it'll cause you problems for a long time. Good luck!
