Learn Hardcore Japanese: gakusei ha mainiti mazime ni nihongo wo benkyousimasu. - Students study Japanese seriously every day.

は marks the topic of the sentence – what we’re talking about.
In this sentence, 学生は means something like “as for the student(s)” or “the student(s) (generally speaking)”.

が usually marks the grammatical subject and often introduces new, specific information.
は often marks known or general information, or the thing you want to set up as the theme of your statement.

So:

学生は毎日真面目に日本語を勉強します。
→ “(As for) the student(s), (they) study Japanese seriously every day.”

If you said 学生が毎日真面目に日本語を勉強します, it would sound more like:
“It is the student(s) who study Japanese seriously every day (as opposed to someone else).” – more specific and sometimes contrastive.

Does 学生 here mean “a student”, “the student”, or “students” (plural)?

Japanese does not mark singular vs plural by default, and it also has no articles like “a” or “the”.
So 学生 on its own can mean:

“a student”
“the student”
“students (in general)”

Which one is correct depends on context in English.

If you’re making a general statement, it’s natural to translate as:
→ “Students study Japanese seriously every day.”
If in context you’re talking about one particular person, you might translate as:
→ “The student studies Japanese seriously every day.”

In Japanese, all of these can be just 学生は.

What exactly does 真面目に (majime ni) mean here? Is it “seriously” or “diligently”?

真面目 (まじめ) as an adjective means “serious”, “earnest”, “diligent”, “conscientious”.

When changed to 真面目に, it becomes an adverb:

“seriously”
“diligently”
“earnestly”

In the context of studying, it suggests:

not slacking off
putting in effort
being disciplined and sincere about it

So depending on nuance, you might translate:

“study Japanese seriously every day”
“study Japanese diligently every day”

Both capture the idea fairly well.

Why does 真面目 become 真面目に? How does that change work grammatically?

真面目 is a na-adjective (な-adjective).

As an adjective before a noun: 真面目な学生 = a serious student
As an adverb modifying a verb: 真面目に勉強します = study seriously/diligently

For na-adjectives, you usually make an adverb by adding に:

静か（な）→ 静かに話す = to speak quietly
元気（な）→ 元気に走る = to run energetically
真面目（な）→ 真面目に勉強する = to study seriously/diligently

So 真面目に is just the adverb form of 真面目.

What is the role of を (o) after 日本語?

を is the particle that marks the direct object of a verb – the thing that the action is done to.

日本語を勉強します
→ literally “study Japanese (language).”

So in this sentence:

日本語 = Japanese (language)
を = marks it as the thing being studied

In English we rely on word order; in Japanese を tells you what is being acted upon.

Why is 勉強します used instead of 勉強する? What’s the difference?

Both come from the verb 勉強する (“to study”).

勉強します is the polite present/future form (ます-form).
勉強する is the plain/casual present/future form (dictionary form).

Use 勉強します:

when speaking politely
to people you don’t know well
in most textbooks and classroom examples

Use 勉強する:

with friends/family
in casual writing (like diaries, notes, many blogs)

So the sentence as written is polite. A casual version would be:

学生は毎日真面目に日本語を勉強する。

Why is the word order 学生は毎日真面目に日本語を勉強します? Could it be rearranged?

Japanese basic word order is S–O–V (Subject–Object–Verb):

(Topic/Subject) + other info + Object + Verb (at the end)

Particles (は, を, に, etc.) mark each word’s role, so you have more flexibility than in English.
You can often move 毎日 and 真面目に around, for example:

学生は毎日真面目に日本語を勉強します。
学生は真面目に毎日日本語を勉強します。
学生は日本語を毎日真面目に勉強します。

All are understandable. The verb still comes at the end, and particles still mark roles, but the order can slightly change emphasis. Beginners are usually taught the “clean” order seen in your sentence.

Could you drop the word 学生 or 毎日 and still have a natural sentence?

Yes. Japanese often omits information that’s clear from context.

If it’s already known who you’re talking about, you can drop 学生は:

(Talking about a certain student)
→ 毎日真面目に日本語を勉強します。
→ “(He/She) studies Japanese seriously every day.”

If the “every day” part is already understood, you could drop 毎日:

学生は真面目に日本語を勉強します。
→ “The student(s) study Japanese seriously.”

The core is usually [topic/subject] + [object + を] + [verb], but in real speech, topics and other obvious elements are frequently left out.

Why does the Japanese sentence not use words like “a” or “the”?

Japanese has no articles like English “a”, “an”, “the”.
Nouns appear bare:

学生 = a student / the student / students
日本語 = Japanese / the Japanese language

Native speakers rely on context, prior conversation, and world knowledge to know whether we mean “a”, “the”, or a generic plural, and translators choose whichever article fits best in English.

So the Japanese sentence is complete without any article-like words.

What does 毎日 (mainichi) do in the sentence, grammatically?

毎日 is a time expression meaning “every day”. In this sentence it works like an adverbial phrase of time, modifying the verb 勉強します:

“study every day”

It answers the question “When?”:

“When does the student study Japanese?”
→ “Every day.”

Time expressions like 毎日, きのう (yesterday), あした (tomorrow) often appear near the beginning of the sentence, but can move around some without changing the core meaning.

Is 学生は毎日真面目に日本語を勉強します talking about a specific student or students in general?

By itself, it’s ambiguous in Japanese and could be either:

Generic statement: “Students (in general) study Japanese seriously every day.”
Specific case: “The (contextual) student studies Japanese seriously every day.”

In many textbook contexts, は + a noun like 学生 often has a generic or habitual nuance, like saying:

犬はよく寝ます。 = “Dogs sleep a lot.” (dogs in general)

So without extra context, it’s natural to interpret your sentence as a general/habitual statement.

Can は here be contrastive, like “as for the student (as opposed to others)”?

Yes, は is often topic-marking, but it can also be contrastive, depending on context.

For example, if the prior sentence was:

友達はあまり勉強しません。
“My friends don’t study much.”

Then:

学生は毎日真面目に日本語を勉強します。
could be heard as:
“But the student studies Japanese seriously every day.”

So は can mean:

neutral topic: “as for the student(s) …”
contrastive topic: “the student(s), on the other hand, …”

The contrast is not in the grammar form itself, but in the surrounding context and emphasis.

How would this sentence change in very casual speech?

Common casual changes:

Use the plain form of the verb:
- 勉強します → 勉強する
Often drop the topic は, if it’s clear who you’re talking about:
- (Talking about a known student)
  → 毎日真面目に日本語勉強する。
を can be dropped in some casual speech, especially in conversation, though keeping it is perfectly fine:
- 日本語を勉強する → 日本語勉強する

So a natural casual version could be:

学生は毎日真面目に日本語を勉強する。 (still explicit topic)
毎日真面目に日本語勉強する。 (very casual, relying on context for the subject)