riidaa ga keikaku wo setumeisita.

リーダー is a loanword from English, meaning leader, so it is written in katakana, the script commonly used for foreign words in Japanese.

A native Japanese alternative might be 指導者 or リーダー役 depending on context, but リーダー is very common in everyday Japanese.

が marks the subject of the sentence: the person or thing that performs the action.

So in:

リーダー が 計画 を 説明した。

リーダー is the one who did the explaining.

In many beginner sentences, が can feel similar to は, but they are not the same:

• が identifies or emphasizes the subject
• は introduces the topic

So リーダーが説明した focuses on the leader as the one who explained.

を marks the direct object, the thing directly affected by the verb.

Here, the verb is 説明した (explained), and the thing being explained is 計画 (the plan), so:

• 計画を説明した = explained the plan

That is why 計画 takes を.

説明した is the past tense plain form of the verb 説明する.

Breakdown:

• 説明 = explanation
• する = to do
• 説明する = to explain
• 説明した = explained

This is a very common pattern in Japanese:

• 勉強する → 勉強した
• 電話する → 電話した
• 説明する → 説明した

These are often called suru-verbs.

Japanese often expresses actions using a noun + する pattern.

So instead of having one simple verb for everything, Japanese frequently uses:

• a noun that names the action
• plus する to turn it into a verb

Examples:

• 説明する = to explain
• 運動する = to exercise
• 予約する = to reserve
• 相談する = to consult

This is completely normal and extremely common in Japanese.

Japanese word order is more flexible than English because particles show each word’s role.

This sentence is:

リーダー が 計画 を 説明した。

A very normal order is:

• subject + が
• object + を
• verb at the end

The most important rule is that the verb usually comes last.

Because the particles mark the roles, you may sometimes see different orders for emphasis, but for learners, this basic pattern is best: subject + object + verb

Yes, it could:

リーダー は 計画 を 説明した。

This would still be natural, but the nuance changes.

• が: focuses on the leader as the one who explained it
• は: presents the leader as the topic, something like As for the leader, he/she explained the plan

So both are possible, but が often sounds more like identifying who did the action.

Japanese usually does not mark the and a directly.

So リーダー could mean:

• the leader
• a leader

The correct interpretation depends on context.

English requires an article, but Japanese often leaves that kind of information unstated unless it is important.

Yes. Very often, Japanese omits the subject when it is clear from context.

So depending on the situation, you might simply hear:

計画を説明した。

This still means (someone) explained the plan, and the listener figures out who from the context.

Japanese often leaves out information that English would usually say explicitly.

It is pronounced roughly like rii-daa ga.

A few helpful points:

• リー is a long rii
• ダー is a long daa
• the particle が here is pronounced ga

So:

• リーダー = rii-daa
• リーダーが = rii-daa ga

The long vowel mark ー makes the vowel sound longer.

No. 説明した is the plain past form.

The polite version would be:

リーダーが計画を説明しました。

So:

• 説明した = plain past
• 説明しました = polite past

The plain form is very common in casual speech, writing, and dictionary-style examples.