Tôi không đủ sức.

Tôi is a neutral, polite way to say I.

It is very common in:

• formal situations
• speaking to strangers
• writing
• situations where you want to sound polite but not overly intimate

However, Vietnamese pronouns depend heavily on age, relationship, and social context. In real life, a speaker might say em, anh, chị, mình, tớ, etc. instead of tôi.

So:

• Tôi không đủ sức. = safe, neutral, correct
• but not always the most natural choice in every relationship

Không is the basic negation word, meaning not.

In Vietnamese, a very common pattern is:

subject + không + predicate

So here:

• Tôi = I
• không = not
• đủ sức = strong enough / have enough strength

That gives:

• Tôi không đủ sức.

This is different from English, which usually needs extra words like do, am, or have.

Đủ means enough, sufficient, or to be enough.

In đủ sức, it means to have enough strength/energy/capacity.

You will see đủ in many similar expressions:

• đủ tiền = enough money
• đủ thời gian = enough time
• đủ lớn = old enough / big enough
• đủ khả năng = capable enough

So đủ is a very useful word for expressing sufficiency.

Sức means strength, energy, stamina, or capacity depending on context.

In đủ sức, it often does not mean only physical muscle strength. It can also mean:

• enough energy
• enough endurance
• enough ability to keep going
• enough mental or emotional strength

So Tôi không đủ sức. can be used:

• literally: I am too weak
• more broadly: I do not have the energy / I cannot go on

Vietnamese often does not need an extra verb like have or to be when the meaning is already clear.

English might say:

• I don’t have enough strength
• I’m not strong enough

Vietnamese can simply say:

• Tôi không đủ sức.

This is very natural. The idea of having enough strength is built into đủ sức, so no extra word is required.

Yes, you can.

Tôi không có đủ sức is also understandable and natural in many contexts. Adding có makes the idea of having more explicit.

Compare:

• Tôi không đủ sức. = concise, natural, very common
• Tôi không có đủ sức. = also natural, slightly more explicit

In many situations, the version without có sounds smoother and more idiomatic.

No. It can be physical, mental, or emotional.

Depending on context, it may mean:

• I’m too weak
• I don’t have the energy
• I can’t keep going
• I’m not able to handle it

If you want to make the meaning more specific, Vietnamese often adds a verb phrase after it:

• Tôi không đủ sức đi tiếp. = I’m not strong enough to continue
• Tôi không đủ sức làm việc nữa. = I don’t have the strength/energy to keep working
• Tôi không đủ sức chịu đựng chuyện này. = I’m not strong enough to endure this

Because in Vietnamese, đủ normally comes before the thing that is enough.

So the structure is:

• đủ + noun

Examples:

• đủ tiền = enough money
• đủ thời gian = enough time
• đủ sức = enough strength

So sức đủ would not be the normal word order here.

The tones are:

• tôi — no tone mark, so it has the level tone (ngang)
• không — also level tone (ngang)
• đủ — hỏi tone
• sức — sắc tone

A rough breakdown:

• tôi: sounds somewhat like toy but not exactly
• không: roughly like khong with a long vowel
• đủ: rising-dipping tone depending on accent
• sức: short, sharp rising tone

If you are learning tones, the two most important things here are:

• đủ and sức must not be said flat
• đ in đủ is not plain English d; it is closer to a firm d sound, while plain d in Vietnamese is pronounced differently

It can absolutely stand alone.

Tôi không đủ sức. is a complete sentence and sounds natural by itself, especially when the context is already known.

For example, if someone asks you to continue, help, or endure something, you can simply reply:

• Tôi không đủ sức.

But Vietnamese also often expands it when the speaker wants to say exactly for what:

• Tôi không đủ sức làm việc này.
• Tôi không đủ sức đi nữa.
• Tôi không đủ sức để tiếp tục.

So both short and longer versions are common.