isya ni syouzyou wo setumeisuru mae ni, memo wo kaite okimasita.

医者に marks the person being addressed by the explanation.

In this sentence, 説明する means to explain, and the pattern is:

• A に B を 説明する = to explain B to A

So:

• 医者に = to the doctor
• 症状を = the symptoms

A native English speaker might expect to here, and に is doing that job.

Because 症状 is the thing being explained, so it takes the direct object particle を.

With 説明する, the usual pattern is:

• someone に
• something を
• 説明する

So:

• 医者に = to the doctor
• 症状を = the symptoms
• 説明する = explain

Even though English says explain something to someone, Japanese marks something with を and someone with に.

Because with 前に meaning before doing, Japanese normally uses the dictionary form of the verb:

• V-る + 前に = before doing V

So:

• 説明する前に = before explaining

This is a very common pattern:

• 食べる前に = before eating
• 寝る前に = before sleeping
• 出かける前に = before going out

Using 説明した前に would sound unnatural here.

前に means before.

In this sentence, it connects the two actions:

1. explaining the symptoms to the doctor
2. writing a memo in advance

So the structure is:

• 説明する前に = before explaining
• メモを書いておきました = I wrote a memo beforehand

It tells you that the memo-writing happened first.

〜ておく often means:

• to do something in advance
• to do something in preparation
• sometimes to leave something as it is

Here, 書いておきました means the speaker wrote the memo ahead of time in preparation for talking to the doctor.

So:

• メモを書きました = I wrote a memo.
• メモを書いておきました = I wrote a memo beforehand / I wrote a memo in preparation.

That extra nuance is important here. The memo was not written randomly; it was written so the later explanation would go more smoothly.

Because the sentence is describing a completed action in the past:

• The speaker wrote the memo.
• That happened before explaining the symptoms.

So 書いておきました is past because the writing is presented as something that was already done.

If you changed it to 書いておきます, it would sound like:

• I will write a memo in advance
• or I write a memo beforehand

That would change the time frame.

In this sentence, メモを書く is best understood as make/write a note rather than a formal business memo.

It suggests the speaker wrote down key information, probably so they would not forget details when talking to the doctor.

So natural English ideas here include:

• I wrote some notes
• I made a note
• I wrote down a memo

Depending on the context, wrote some notes beforehand is often the most natural translation.

The comma separates the time clause from the main action:

• 医者に症状を説明する前に、
• メモを書いておきました。

This is similar to English writing:

• Before explaining my symptoms to the doctor, I wrote a memo.

The comma is helpful because the first part is a longer introductory phrase. It is not absolutely tied to grammar in the same rigid way as in English, but it makes the sentence easier to read.

Normally, no—not in this sentence.

With 説明する, the listener/recipient is usually marked by に, not へ:

• 先生に説明する
• 友だちに説明する
• 医者に説明する

へ is more associated with direction or destination, like to/toward a place. Since the doctor here is the recipient of the explanation, に is the natural choice.

A good breakdown is:

• 医者に = to the doctor
• 症状を = the symptoms
• 説明する = explain
• 前に = before
• メモを = a memo / notes
• 書いておきました = wrote beforehand / wrote in preparation

So the structure is:

[Before doing X], [did Y beforehand].

More specifically:

• [医者に症状を説明する前に]
• [メモを書いておきました]

This kind of sentence is very common in Japanese: a subordinate clause first, then the main clause at the end.

Often, yes.

Japanese nouns usually do not explicitly show singular vs. plural the way English does. So 症状 can mean:

• a symptom
• symptoms

In this context, English naturally tends to say symptoms, because people usually explain multiple details to a doctor.

So although the Japanese does not force plurality grammatically, the natural English meaning is often symptoms.

Yes, and that is probably the most natural interpretation.

Because the sentence says:

• Before explaining my symptoms to the doctor, I wrote a memo beforehand

the memo is likely something the speaker made for reference, so they would remember what to say.

It does not necessarily mean the doctor read the memo. It more likely means:

• the speaker organized their thoughts
• the speaker did not want to forget important details
• the memo was part of preparation

That nuance comes especially from 書いておきました.