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Disclaimer: The grammatical rules discussed in this article are personal abstractions and do not have any authoritative references. There may be inaccuracies or errors; this is for reference only.

1. Review of Traditional Grammatical Rules of "doing"

In traditional teaching, teachers tell us to use "doing" in the following situations:

• When a verb functions as a noun, it should be transformed into the gerund (doing) form: For example, "take" becomes "taking."
• Present continuous tense uses "be doing," which can indicate more than just "currently doing." Futhermore, "be doing" can express "something happening at this stage," "emotional emphasis", "repeated actions," and more.
• After a preposition, use +doing.
• Various other special cases.

Just remember this: when a verb functions as a noun, it should be changed to the "doing" form; everything else can be disregarded.

In other words, when you want to express sth, use "doing."

2.Using "doing" When Expressing something

sth refers to "something," in my rules, which can mean a noun, an adjective, or a sentence.

For example:

• I like apples. : "apples" is a sth.
• I like eating apples.:"eating apples" is a sth.

You can regard sth as a noun. We know that when a verb needs to function as a noun, it should be changed to the "doing" form.

Thus, in a sentence, when you need to use "sth" and it starts with a verb, you must use "ing." This is my simple rule for using "doing".

Next, I will explain various situations where "doing" is used to help you understand the above rule. Once you grasp it, you can confidently handle when to use "doing."

3. Various Situations for "doing"

3.1 Present Continuous Tense: be doing

The verb "be" in English indicates "the state of ... is ..."

A is sth, which means the state of A is sth.

For example:

• Tom is happy: Tom's state is happy.
• A bird is on the tree: The bird's state is "on the tree."

When sth refers to "doing something," it indicates: A's state is "doing something." Therefore, "be doing" means currently doing, which is what we call the present continuous tense.

For example:

• Tom is doing homework: Tom's state is "doing homework."
• A bird is singing: The bird's state is "singing." Here, "singing" can be understood as a gerund or directly as a noun, and both interpretations are correct.

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"Be doing" can indicate "present continuous tense," but it can also represent many other situations. However, I believe they all express A is sth (A's state is sth).

(1) "Be doing" indicates something that hasn’t happened at the current moment but has been ongoing recently.

• I am studing for my exams these days. : My state is "studying for exams these days."
• They are living in a hotel while they look for a new house. :Their state is "living in a hotel while looking for a new house."

(2) "Be doing" indicates future plans or arrangements.

• We are meeting the clients tomorrow morning. :Our state is "meeting the clients tomorrow."
• They are moving to a new house next month. : Their state is "moving next month."

If you use "future tense" to indicate future plans or arrangements, it emphasizes "this will be done later." In contrast, "be doing" emphasizes "the state," implying that "this plan is already confirmed."

(3) "Be doing" indicates repeated actions during a certain time period.

• She is always helping others with their work. :Her state is "always helping others."
• She is always borrowing my clothes without asking! :Her state is "always borrowing my clothes without asking!"

Here, the "state" can be understood as "personal character".

Summary: Whenever sth needs to follow "be," feel free to use "be doing."

3.2 Using "doing" after Prepositions

Teachers always tell us to use "doing" after prepositions. Words like "on," "at," "in," and "about" are prepositions, and the "to" in "look forward to" is also a preposition. With so many prepositions, do I have to memorize them all?

Actually, it’s simple: Regardless of whether it’s a preposition, if sth follows, use "doing." A few examples make this clear:

(1) A is talking about sth

• Tom is talking about his friends. : "sth" is "his friends."
• Tom is talking about going on vacation. : "sth" is "going on vacation."

(2) A is interested in sth

• He's interested in movies. : "sth" is "movies."
• He's interested in learning English. :"sth" is "learning English."

(3) A is looking forward to sth

• I'm looking forward to your reply. : "sth" is "your reply."
• I'm looking forward to receiving your reply. : "sth" is "receiving your reply."

(4) sth when sth

• I will leave when I finish it. : "sth" is "I finish it".
• I will leave when finishing it. :"sth" is "finishing it".

(5) sth after sth

• I will leave after I finish it.
• I will leave after finishing it.

Summary: If sth follow any sentence or word, then use "doing" form.

3.3 Using "doing" after Verbs (Gerund Form)

Teachers tell us that when a verb is followed by another verb, you can use either "to do" or "doing."

The reason is simple: a verb needs to be followed by sth.

(1) A likes sth.

• Tom likes apples. : "sth" is "apples."
• Tom likes playing basketball. : "sth" is "playing basketball."
• Tom likes to play basketball. : "sth" is "to play basketball."

3.4 Using "doing" in Object Complements

Teachers tell us that when a verb acts as an object complement, the "doing" form of the verb should be used.

The essence is that the object complement is sth.

• They painted the wall blue :"sth" is "blue."
• We consider him a genius: "sth" is "a genius."
• I saw him swimming in the lake. : "sth" is "swimming in the lake."

Note: Using "doing" in object complements has few exceptions in English. They are: help, make, let, have. When these verbs are used, "doing" is not needed.

(1) I help him do his homework.

(2) I found her crying because she made him cry.

(3) They let me join the club.

(4) I had the mechanic fix my car.

4. Comparison Between "to do" and "doing"

This article emphasizes that when sth is an action, "doing" should be used.

However, besides "present continuous tense," "to do" can also be used in these cases.

The differences are:

• doing:emphasizes "in general (usually)" or "currently doing."
• to do:emphasizes "future action," "purpose," or "intention."

If your sth does not need to emphasize the above points, then using "to do" or "doing" is perfectly acceptable.

(1) Doing indicates "usually" meaning::

• I enjoy doing my homework. :I (usually) enjoy doing homework
• I don't mind doing the dishes. : I (usually) don’t mind washing dishes.

(2) Doing indicates something ongoing

• He is busy doing his homework. : He is (currently) busy doing homework.

(3) To do indicates "future action"

• She has a lot to do today.
• They plan to do a project together.

(4) To do indicates purpose and intention

• I want to do my best.

(5) If none of the above points need emphasis, either "to do" or "doing" can be used:

• I love doing yoga.
• I love to do yoga.