How does one differentiate between future tense (will) and present tense?


How do I differentiate between future tense and present tense. For example,
the sentence:  

> 私は魚を食べる

If I am not mistaken, the above sentence can either mean "I **will** eat fish"
_or_ "I eat fish".

Also, if I am not mistaken, conjugating the verb to the ます form only aims to
make the sentence polite and doesn't actually change the tense or meaning. For
the case of the example above, the sentence becomes 私は魚を食べます, which can again
either mean "I **will** eat fish" _or_ "I eat fish" in polite form.

I have read that generally, context will tell me whether the sentence is
referring to the present or future tense. But take the example above, from
just that one sentence (assuming someone tells me just that one sentence in
the example), how do I determine what the tense of the sentence is?




I think @Eiríkr Útlendi's answer gives a good general answer.

If we look at `私は魚を食べる` specifically: if it's said without anything else, I
would interpret it as an explanation of what the speaker does (usually) ("I
eat fish (in general)"), because it's uncommon to say 私は if you are not
generalizing.

If it's literally only `魚を食べる`, I would think it's creating a context for what
comes next in a narration etc. (a rhetorical technique). E.g.:
`魚を食べる。自然との対話である。`. This is because not abbreviating を is uncommon if you are
just stating your intent (but it is common in the mentioned context).

If you want to say you are eating a fish right now, then it would be
`魚を食べている`. If someone wants to say they will eat a fish, they would say `魚食べる`
(which is equivalent to `魚を食べる` - but abbreviating を is more common in this
context). This will imply it's in the future (as you are not eating the fish
right now).

So in conclusion: the most frequent form you hear would be `魚食べる`, and if I
hear that in complete isolation my bet would be that the person intends to eat
a fish in the near future. The next likely meaning would be a general
statement, like if they were asked a question (「アザラシって何食べるの？」）.




> If I am not mistaken, the above sentence can either mean "I **will** eat
> fish" _or_ "I eat fish".

Correct.

> ... conjugating the verb to the ます form only aims to make the sentence
> polite and doesn't actually change the tense or meaning.

Correct. ます is a conjugation form that affects the social context of the
sentence (indicating details about who is speaking and who is listening, and
also sometimes who the sentence is about), but not the tense (i.e. the time
when the action occurs) or aspect (i.e. the completedness, repetitiveness,
etc. of the action).

> ... from just that one sentence (assuming someone tells me just that one
> sentence in the example), how do I determine what the tense of the sentence
> is?

Japanese **has no future tense**. It just doesn't exist as a feature of the
language. Japanese is often described as having a **past tense** , where verbs
end in _-ta_ , and a **non-past tense** , where verbs end in _-u_. So in
answer to your question, that one sentence is in the **non-past tense**.

Note that **past tense** itself is sometimes the wrong term for words ending
in _-ta_. Consider a sentence like: 明日、このプロジェクトが‍ **終わった** ‍後、（なにか）しましょう /
"Tomorrow, after this project **has finished** , let's do (something)." Here,
終わった clearly has the _-ta_ ending, but we're talking about tomorrow, so it
_can't_ be about the past. This is where we instead have to talk about
**aspect** : in this case, the _-ta_ ending doesn't describe **tense** , or
_when_ the action happens, but rather **aspect** , which could be various
other dimensions of verb-ness: completion, repetition, ongoing state, so-
called "telic" action with distinct before-and-after states (like "change" or
"pick up") or "atelic" action with no such clear distinction (like "work" or
"sleep"), etc. etc.

More at Wikipedia about [grammatical
aspect](https://en.wikipedia.org/wiki/Grammatical_aspect), [grammatical
tense](https://en.wikipedia.org/wiki/Grammatical_tense), and the [non-past
tense](https://en.wikipedia.org/wiki/Nonpast_tense).

### Update

In response to a comment, let's look again at the core questions:

> How do I differentiate between future tense and present tense?

Context. Grammatically, there is no difference between future and present
tense in Japanese. See the older portions of this post above for further
details on that.

> But take the example above, from just that one sentence (assuming someone
> tells me just that one sentence in the example), how do I determine what the
> tense of the sentence is?

You don't.

If you're translating into English, your options are to either pick a tense,
or translate in both tenses and add an explanatory note.

