tiānqì gǎibiàn le wǒmen de jìhuà.

The sentence follows the normal Chinese pattern:

Subject + Verb + 了 + Object

So here:

• 天气 = the weather
• 改变了 = changed
• 我们的计划 = our plan / our plans

So literally, it is:

The weather changed our plan(s).

Chinese usually keeps this straightforward SVO order, much like English.

Here 了 marks the action as completed or as a new fact/result.

So:

• 改变 = change / to change
• 改变了 = changed / has changed

In this sentence, 了 tells you that the weather’s effect on the plan has already happened.

Without 了, the sentence would sound less like a completed event and more like a general statement, a possibility, or something depending on context.

的 links a possessor to a noun.

So:

• 我们 = we / us
• 我们的 = our
• 我们的计划 = our plan / our plans

In Chinese, pronouns usually need 的 before a noun when showing possession:

• 我的书 = my book
• 你的老师 = your teacher
• 我们的计划 = our plan(s)

So 我们计划 would normally not work here if you mean our plan.

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

So 计划 can mean:

• plan
• plans

The exact meaning depends on context.

That means 我们的计划 could be:

• our plan
• our plans

If the broader context makes it clear that there was only one plan, you understand it as singular. If there were several, you understand it as plural.

Yes. In this sentence, 改变 is a transitive verb, which means it takes an object.

• 天气 = subject
• 改变了 = verb
• 我们的计划 = object

So the weather is changing something: our plan(s).

That is different from a sentence where something simply changes by itself.

For example:

• 天气变了。 = The weather changed.
• 天气改变了我们的计划。 = The weather changed our plans.

In your sentence, the weather is not just changing; it is causing a change to something else.

You can, but the meaning and feel change.

• 天气改变了我们的计划。 = The weather changed our plans.
• 天气改变我们的计划。 = The weather changes our plans / the weather changes our plans in general / the weather is changing our plans, depending on context

Without 了, it sounds less clearly like a completed event. In a sentence about a specific situation that already happened, 了 is the natural choice.

Yes, it is understandable and grammatical.

However, depending on context, native speakers might also use other verbs that sound more specific or natural in everyday conversation, such as:

• 天气影响了我们的计划。 = The weather affected our plans.
• 天气打乱了我们的计划。 = The weather disrupted our plans.
• 天气的变化改变了我们的计划。 = Changes in the weather changed our plans.

Your sentence is fine, but in real life people often choose a verb that shows the exact kind of effect more clearly.

Because the sentence is focusing on what caused the change.

In:

天气改变了我们的计划。

the topic is the weather as the thing that caused the result.

If you wanted to focus more on what we did, Chinese might say something like:

• 因为天气，我们改变了计划。 = Because of the weather, we changed the plan.

Compare the two:

• 天气改变了我们的计划。 = the weather changed our plans
• 因为天气，我们改变了计划。 = because of the weather, we changed the plans ourselves

Both are possible, but the focus is different.

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

So nouns like 天气 and 计划 appear without article words.

English needs to choose between:

• the weather
• a plan
• our plans

Chinese usually leaves that information to context, possession words, or other modifiers.

So 我们的计划 already feels definite because our makes it specific, even without an article.

A natural pronunciation is:

tiānqì gǎibiàn le wǒmen de jìhuà

A few helpful points:

• 天气 (tiānqì): first tone + fourth tone
• 改变 (gǎibiàn): third tone + fourth tone
• 了 (le): neutral tone here
• 我们 (wǒmen): third tone + neutral tone
• 的 (de): neutral tone
• 计划 (jìhuà): fourth tone + fourth tone

In natural speech, 了, 的, and 们 are light and unstressed.

So it sounds something like:

tiān-qì gǎi-biàn le wǒ-men de jì-huà

with the main stress usually falling more on content words like 天气, 改变, and 计划.