Learn Hardcore Japanese: kyou no siai ha omosirokatta desu.

This is just a conventional spelling rule that has been preserved from older stages of the language. So in 今日の試合は面白かったです, the は after 試合 is the topic particle, and you read it as wa, not ha:

しあいは → shiai wa
not shiai ha.

What does the particle の do in 今日の試合?

の here is a connecting/possessive particle that links 今日 (today) to 試合 (match/game).

It has a function similar to:

English “’s”: 今日の試合 ≈ “today’s game”
Or English “of”: 今日の試合 ≈ “the game of today”

So:

今日 = today
試合 = match / game
今日の試合 = the match that happened today / today’s match.

What does the particle は do in 試合は? Why not が?

は marks the topic of the sentence — what we’re talking about.

今日の試合は…
= “As for today’s game, …” / “Speaking of today’s game, …”

Using が instead:

今日の試合が面白かったです would more strongly mark 今日の試合 as the subject that was interesting, often in contrast to something else (for example, “It was today’s game (and not yesterday’s) that was interesting”).

Basic nuance:

AはB – “As for A, (it) is B.” (topic, general comment)
AがB – “A is B.” (focus on A itself, often new or contrastive information)

In everyday speech about an already-known thing like “today’s game,” は sounds more natural because you’re just making a comment about it.

Why do we need です after 面白かった? Isn’t 面白かった already a complete form?

Grammatically, 面白かった by itself is a complete predicate:

面白かった。 = “(It) was interesting/fun.”

However, adding です:

面白かったです。

does two main things:

Politeness:
です is the polite copula. Attaching it makes the sentence polite style (丁寧語).
- 面白かった。 → plain / casual
- 面白かったです。 → polite / appropriate for most everyday formal interactions
Softening:
It can make your statement sound a bit softer and more neutral in tone.

It’s not “double past.” 面白かった carries the past tense; です just adds politeness.

How is the past tense formed in 面白かった? What is the dictionary form?

The adjective here is an い-adjective:

Dictionary form: 面白い (omoshiroi) = interesting / fun

To make the past tense of an い-adjective:

Remove the final い
- 面白い → 面白
Add かった
- 面白 + かった → 面白かった

So:

面白い = is interesting
面白かった = was interesting

This same pattern applies to most い-adjectives:

高い (takai, expensive/high) → 高かった (was expensive/high)
新しい (atarashii, new) → 新しかった (was new)

Why is the verb/adjective phrase 面白かったです at the end? Could I say it earlier in the sentence?

Japanese is broadly SOV (Subject–Object–Verb), or more accurately, the main predicate comes at the end.

In 今日の試合は面白かったです:

今日の試合は = topic (“as for today’s game”)
面白かったです = predicate (“was interesting/fun”)

The natural, neutral order is to place the main predicate at the end of the sentence. You cannot normally move 面白かったです somewhere in the middle like:

✕ 今日の面白かったです試合は
This is ungrammatical.

You can, however, move or omit other parts in context (especially the topic) but the final predicate generally stays at the end.

Can I drop some parts and still be correct? For example, could I just say 面白かったです?

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

Depending on what is already understood, you can say:

面白かったです。
“(It) was interesting/fun.”
– If it’s obvious you’re talking about “today’s game,” this is natural.
今日の試合は面白かった。
(Dropping です) – casual speech, especially with friends.
今日の試合、面白かった。
(Dropping は) – in casual speech, the は can be omitted after a clear topic, especially in conversation.

The full 今日の試合は面白かったです。 is just the most explicit and polite version.

What’s the difference in tone between 面白かったです and 面白かった?

Both mean “was interesting/fun,” but:

面白かったです。
- Polite speech (丁寧語)
- Suitable for talking to people you’re not close to, superiors, customers, etc.
面白かった。
- Plain/casual speech
- Used with friends, family, or when you’re thinking to yourself

Politeness level affects the relationship and social distance, not the basic meaning.

Why is 試合 used instead of ゲーム? Don’t both mean “game”?

Both can translate as “game,” but their nuance and usage differ:

試合 (しあい)
- A match / contest / bout, usually in sports, martial arts, or competitions
- Has a more formal / “official match” feel
- e.g. サッカーの試合 (a soccer match), 野球の試合 (a baseball game)
ゲーム (げーむ)
- A game in a broader sense: video game, casual game, board game
- Also used in some sports contexts, but often more loosely

In something like a sports context (baseball, soccer, etc.), 今日の試合 naturally means “today’s match/game (that was played).” Saying 今日のゲーム would usually be understood, but 試合 sounds more like an actual organized sporting match.

Is 今日の試合は面白かったです talking about the past, present, or future?

It’s clearly about the past:

The event: 今日の試合 – the match that occurred today (already over at the time of speaking)
The adjective: 面白かった – past tense form of 面白い

So the meaning is “Today’s game was interesting/fun.”
The word 今日 (today) alone doesn’t mark tense; the past form 〜かった on the adjective does.

If I know the subject is “today’s game,” why don’t I need a word like “it” in Japanese?

Japanese usually omits pronouns like “I,” “you,” “it” when the referent is obvious.

In English you must say:

“Today’s game was interesting.” ↑ you need a subject (“game”) and a verb (“was”).

In Japanese, the topic–comment structure works differently:

今日の試合は – sets the topic (“as for today’s game”)
面白かったです – comments on that topic (“was interesting”)

There’s no separate word for “it”:

今日の試合は面白かったです。
Literally: “As for today’s game, (it) was interesting.”

The “it” is already understood from the topic 今日の試合.