Learn Hardcore Japanese: kinou ha ryouribon de satou to sio no irekata wo sirabemasita. - Yesterday I looked up how to add sugar and salt in a cookbook.

So 昨日は emphasizes “yesterday” as the context of the statement: as for what I did yesterday, I looked this up in a cookbook. It’s stylistically very common and natural.

What does 料理本で mean, and why is the particle で used instead of に or から?

Here で marks the means / medium used to do the action.

料理本で = “by means of a cookbook,” “using a cookbook,” “in a cookbook (as the reference source)”.

Common uses of で you see here:

Place where an action happens
- 学校で勉強します。= “I study at school.”
Tool / means / medium
- パソコンで調べます。= “I look it up on a computer.”
- 料理本で調べました。= “I looked it up in a cookbook (using a cookbook).”

Using に here (料理本に調べました) would be wrong. に is not used to mark the means/tool of investigation like this.
から (料理本から調べました) would sound odd; から would emphasize “from” as a source of information, but with 調べる that’s not idiomatic. で is the natural choice.

In 砂糖と塩の入れ方, what exactly are と and の doing? How is this phrase structured?

Break it down like this:

砂糖と塩 = “sugar and salt” (と connects two nouns as “A and B”).
塩の入れ方 = “the way of putting in salt.”
When you say 砂糖と塩の入れ方, the whole 砂糖と塩 is treated as one unit that modifies 入れ方 with の.

So structurally:

[砂糖と塩] の [入れ方]
“the way of putting in sugar and salt”

の here is linking “sugar and salt” (what you are putting in) to 入れ方 (“way of putting in”). It’s like saying:

“the method of adding sugar and salt,”
“how to put in sugar and salt.”

What does 入れ方 mean? How does this verb stem + 方 pattern work?

入れ方 comes from:

verb 入れる (“to put in; to add”)
take the ます-stem: 入れ
add 方（かた）: 入れ方

Verb-stem + 方 means “the way of doing [verb]” / “how to [verb]”.

Examples:

使い方 (使う → 使い) = “how to use,” “way of using”
書き方 (書く → 書き) = “how to write,” “writing style / method”
読み方 (読む → 読み) = “how to read,” “pronunciation (for kanji etc.)”

So 入れ方 literally = “the way of putting in,” “how to put in / how to add.”

In the sentence, 砂糖と塩の入れ方 = “how to add sugar and salt.”

Why is 入れ方 followed by を? What is を doing in this sentence?

を marks the direct object of the verb 調べました (“looked up / investigated”).

何を調べましたか。 = “What did you look up?”
Answer: 砂糖と塩の入れ方を調べました。
“I looked up how to add sugar and salt.”

So:

入れ方 = the thing you’re investigating
入れ方を調べました = “(I) researched / looked up the way (to do it).”

Even though 入れ方 is a “way of doing” expression, grammatically it’s just a noun, so it can take を as a normal direct object.

What nuance does 調べました have? How is it different from 探しました or 勉強しました?

調べる means to check / look up / investigate information, usually in a source like a book, the internet, a dictionary, etc.

調べました (polite past) = “(I) looked up / checked / investigated.”

Compare:

探す = “to look for, search for (a thing you don’t have yet, or can’t find).”
- 本を探しました。= “I searched for the book (physically or online).”
勉強する = “to study (a subject, over time).”
- 日本語を勉強しました。= “I studied Japanese.”

In this sentence:

料理本で調べました。 = “I looked up (the information) in a cookbook.”
You already have the book; you’re checking its contents.

If you said:

料理本で勉強しました。 = “I studied using a cookbook.”
→ more like systematic study, not just checking one specific thing.
料理本を探しました。 = “I searched for a cookbook.”
→ looking for the book itself, not for information inside it.

So 調べました is the most natural verb here.

Can the word order change? For example, could I say 料理本で昨日は砂糖と塩の入れ方を調べました? How flexible is it?

Japanese word order is relatively flexible as long as:

the verb comes at the end, and
particles clearly mark each element’s role.

Natural variations include:

昨日は料理本で砂糖と塩の入れ方を調べました。 (original)
砂糖と塩の入れ方を料理本で昨日は調べました。 (a bit marked/emphatic)
料理本で昨日は砂糖と塩の入れ方を調べました。 (possible, but sounds more like you’re emphasizing “using a cookbook, yesterday…”)

Most neutral and common is to put:

Time (昨日は) near the beginning,
Then place/means (料理本で),
Then object (砂糖と塩の入れ方を),
Then the verb (調べました).

Other orders can be used for emphasis or in speech, but the original is the cleanest, most natural basic pattern.

Could I say 砂糖や塩の入れ方 instead of 砂糖と塩の入れ方? What’s the difference between と and や here?

Both are grammatically possible, but the nuance changes:

砂糖と塩の入れ方
- と = “and” for a definite, complete list.
- Implies you specifically mean sugar and salt (these two items).
砂糖や塩の入れ方
- や = “and ~, etc.” for a non-exhaustive list.
- Implies “ways of adding things like sugar, salt, etc.” — maybe there are other seasonings too.

In a context where you really are just talking about those two specific ingredients, 砂糖と塩の入れ方 is more straightforward and natural.

Could I omit some particles here, like は or を? Would the sentence still be okay?

In casual spoken Japanese, some particles can be dropped, but it changes the feel and can affect clarity.

Possible colloquial forms:

昨日料理本で砂糖と塩の入れ方調べました。
- Drops は and を.
- Sounds casual and spoken, but still understandable because the roles are clear from position/context.

However:

In standard written or polite speech (like your example with 調べました), it’s better to keep the particles:
- 昨日は … (topic)
- 入れ方を (direct object)

So yes, you can drop them in informal speech, but the original with particles is the correct, textbook‑natural form, especially in polite style.

What level of politeness is 調べました? How would this sentence look in casual form?

調べました is the polite past form (ます-form). The whole sentence is at polite level.

Casual (plain) form:

昨日は料理本で砂糖と塩の入れ方を調べた。

Changes:

調べました → 調べた (plain past)
Everything else is the same.

You’d use the polite version 調べました when speaking to strangers, teachers, superiors, or in formal writing.
The casual 調べた is for friends, family, or informal contexts.