Does anyone know a good app or website or have tips to learn Korean sentence particles and grammar rules?
I have trouble with 은, 는, 가, 이, and gender pronouns.

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It's natural that English(a inflectional language) speakers have difficulties with the particles in Korean language (an agglutinative language). Native Koreans, of course, use them automatically, but explaining it is different thing. I found good explanation below from internet.
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Topic Marker 은/는 vs. Subject Marker 이/가

1. Topic Marker 은/는: Old Information
Subject Marker 이/가: New Information
이 / 가 is used to ask about new information and 은 / 는 is used to answer the question.

Q :이름이 뭐예요? What is your name?
A : 제 이름은 이민재예요. My name is Minjae Lee.

Q : 오늘이 무슨 요일이에요? What day is it today?
A : 오늘은 토요일이에요. Today is Saturday.

Also, 이 / 가 can be used when something is mentioned first and 은 / 는 is used to indicate what you have told.

테이블 위에 사과가 있어요. 그 사과는 맛있어 보여요.
There is an apple on the table. The apple looks delicious.

2. Topic Marker 은/는: General or Factual Statement
Subject Marker 이/가: Specific Statement
The topic marker 은 / 는 is used when you make general or factual statements, whereas the subject marker 이 / 가 is not.

For example,
치타는 가장 빠른 동물이다. Cheetah is the fastest animal.

However, if you saw a cheetah moving very slowly, you might say,
치타가 느리다. (That) cheetah is slow.

3. Topic Marker 은/는: Contrast or Comparison
Subject Marker 이/가: X
은 / 는, depending on context, can be used to compare or contrast two things.

지영은 사과를 좋아해요. 그런데 수지는 사과를 안 좋아해요. Jiyoung likes apples, but Suji doesn’t like apples.

혜미가 사과는 좋아해요. 그런데 바나나는 안 좋아해요.
Hyemi likes apples, but she doesn’t like bananas.

오늘은 일 안 해도 돼요. 그런데 내일은 일 해야 돼요.
Today I don’t have to work. However, tomorrow I have to work.


4. Topic Marker 은/는: Focus on Description
Subject Marker 이/가: Focus on Subject
As you can guess from the name, subject marker 이 / 가 focuses on the subject and topic marker 은 / 는 focuses on the topic, in other words, action or description of the subject.

That’s why you always say “누가 (who)? ” when you want to know who has done something. (누가 is an abbreviation of 누구 + 가.)

누가 아직 안 왔어? Who’s still not here?

Let’s see the difference with these conversations.

A : 누가 설거지 했어? Who did the dishes?
B : 수지가 했어. Suji did.

In the first conversation, person A is curious about the person who did the dishes. So, the focus of the answer is the subject. That’s why subject marker 가 is used twice, 누가 and 수지가.

A: 수지는 지금 뭐해요? What is Suji doing now?
B: 수지는 지금 쇼핑하고 있어요. Suji is shopping now.

In the second conversation, person A is wondering what Suji is doing now. So, the focus of the answer is the action. That’s why topic marker 는 is used. As you guys have noticed, there are quite a few differences.