wǒ juédìng gǎibiàn jìhuà.

In Mandarin, you usually do not need a separate word like English to after 决定.

So:

• 我决定改变计划 = I decided to change the plan / I decide to change the plan

The structure is simply:

• 我 = I
• 决定 = decide
• 改变计划 = change the plan

Mandarin often puts one verb directly after another when the first verb describes a decision, desire, ability, or intention.

The basic structure is:

Subject + Verb 1 + Verb 2 + Object

So here:

• 我 = subject
• 决定 = Verb 1, showing the decision
• 改变 = Verb 2, the action being decided on
• 计划 = object of 改变

So 改变计划 is a verb-object phrase meaning change the plan, and 决定 tells you that this is the action the speaker decided to do.

Here, 决定 is a verb meaning to decide.

But 决定 can also be a noun meaning decision, depending on the sentence.

Examples:

• 我决定改变计划。
  决定 = verb = decide

• 这是我的决定。
  决定 = noun = decision

In your sentence, because 决定 comes right after the subject 我 and before another action, it is clearly being used as a verb.

This is very normal in Mandarin.

The first verb, 决定, tells you about the speaker’s mental action: decide.
The second verb, 改变, tells you the actual action: change.

So the sentence is not strange at all. It works like:

• I decide
  • change the plan

In natural English we insert to, but Mandarin does not need that.

You can think of it as:

• 我决定 = I decided
• 改变计划 = to change the plan

It can mean either plan or plans, depending on context.

Mandarin nouns usually do not change form for singular or plural the way English nouns do.

So:

• 改变计划 could mean change the plan
• or change plans

If the context makes the number important, Mandarin can add words to clarify, such as:

• 这个计划 = this plan
• 那些计划 = those plans
• 很多计划 = many plans

In your sentence, English often translates it as the plan, but the Chinese itself does not explicitly mark the.

Mandarin does not use articles like a, an, and the the way English does.

So 计划 by itself can mean:

• a plan
• the plan
• plans

The exact meaning depends on context.

If you want to be more specific, you can say:

• 这个计划 = this plan
• 那个计划 = that plan

But in many sentences, just 计划 is enough.

You might hear forms like that, but for a learner, 我决定改变计划 is the safest and most natural basic sentence.

Why? Because 了 can be tricky.

• 我决定改变计划 can already mean I decided to change the plan if the context is past.
• Mandarin often relies on context or time words instead of always marking tense.

If you want to make the completed decision clearer, native speakers more often say things like:

• 我决定改变计划了。
• 我已经决定改变计划了。

These feel more natural than inserting 了 directly after 决定 in many contexts.

So for now:

• 我决定改变计划 = excellent standard form

You put 不 before the action being decided on:

• 我决定不改变计划。
  = I decided not to change the plan

This is different from:

• 我不决定改变计划。

That second sentence would mean something more like I do not decide to change the plan, which is usually not what you want.

So the key idea is:

• 决定不 + verb = decide not to do something

Yes, it is a very common and natural combination.

• 改变 = to change, alter
• 计划 = plan

So 改变计划 is a normal verb-object phrase meaning change the plan or change plans.

You will also see similar patterns like:

• 改变想法 = change one’s mind
• 改变方法 = change the method
• 改变方向 = change direction

Yes.

Depending on context:

• 决定 can mean decide, make up one’s mind, or sometimes determine
• 改变 can mean change, alter, or modify

So 我决定改变计划 might be translated as:

• I decided to change the plan.
• I’ve decided to change the plan.
• I decided to alter the plan.
• I made up my mind to change the plan.

The exact English wording depends on context and tone.

The pinyin is:

wǒ juédìng gǎibiàn jìhuà

Tone by tone:

• 我 wǒ = 3rd tone
• 决 jué = 2nd tone
• 定 dìng = 4th tone
• 改 gǎi = 3rd tone
• 变 biàn = 4th tone
• 计 jì = 4th tone
• 划 huà = 4th tone

A natural rhythm is:

wǒ | juédìng | gǎibiàn jìhuà

A couple of pronunciation notes:

• jué is not like English joo-ay. The jü-type sound is made with the tongue forward, like j plus a rounded ü sound.
• huà starts with a light Mandarin h, not an English w sound.

Often, yes, context helps decide the time.

Mandarin verbs do not change form the way English verbs do. So 决定 can mean:

• decide
• decided
• have decided

depending on the situation.

For example:

• 昨天我决定改变计划。
  = Yesterday I decided to change the plan.

• 现在我决定改变计划。
  = Now I decide to change the plan.

• 我已经决定改变计划。
  = I have already decided to change the plan.

So the sentence by itself is grammatically complete, but context tells you the time.

Yes, in many everyday situations, 改 is very common and often sounds more conversational.

So you can say:

• 我决定改计划。

That is shorter and natural in speech.

But 改变计划 sounds a bit more neutral or formal, and it is a very good form for learners to know.

A rough comparison:

• 改计划 = more casual, everyday
• 改变计划 = slightly fuller, more formal or careful

Both are correct.