Это обстоятельство изменило план.

Because обстоятельство is a neuter noun. In Russian, this changes form to match the noun’s gender and case:

• masculine: этот
• feminine: эта
• neuter: это

So with обстоятельство, the correct form is это обстоятельство.

A very common clue is the ending. Nouns ending in -о or -е are often neuter, and обстоятельство ends in -о.

That is why:

• обстоятельство = neuter
• the past-tense verb is also neuter: изменило

In the past tense, Russian verbs agree with the subject in gender and number.

Here the subject is обстоятельство, which is neuter singular, so the verb takes the neuter singular past ending:

• masculine: изменил
• feminine: изменила
• neuter: изменило
• plural: изменили

So Это обстоятельство изменило план is grammatically matched.

It is in the nominative case, because it is the subject of the sentence — the thing doing the action.

So:

• Это обстоятельство = the subject
• изменило = the verb
• план = the object

План is in the accusative case, because it is the direct object of the verb.

But for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular. So:

• nominative: план
• accusative: план

The case is still accusative — it just happens to have the same form.

Изменило is the perfective past form of изменить.

Perfective verbs usually show a completed action or a result. In this sentence, the idea is that the circumstance caused a change in the plan — a finished result.

Compare:

• изменило = changed, brought about a change
• изменяло = was changing / used to change / changed repeatedly

So изменило is the natural choice if you mean one completed change.

Here это is functioning like a demonstrative determiner: it means this and modifies the noun обстоятельство.

So in это обстоятельство, it is not standing alone as this/it. It is directly attached to the noun, like English this circumstance.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Это обстоятельство изменило план.

But other orders are possible for emphasis, for example:

• План изменило это обстоятельство.
  This can emphasize the plan or contrast what changed it.

Even when the order changes, the cases still tell you who did what.

No. Обстоятельство has a normal everyday meaning too: circumstance, factor, or condition depending on context.

Learners often meet it in grammar first, where обстоятельство means an adverbial modifier. But in this sentence it is just the ordinary lexical meaning: a circumstance or factor.

A natural pronunciation is:

Э́то обстояте́льство измени́ло план.

Approximate stress:

• Э́то
• обстояте́льство
• измени́ло
• план

A rough English-style approximation would be: EH-ta ab-sta-YA-tel-stva iz-me-NEE-la plan

But it is best to listen to native audio if possible, especially for the long word обстоятельство.

Russian has no articles like a or the.

So план can mean:

• a plan
• the plan

The exact meaning depends on context. In this sentence, English may naturally translate it as the plan, but Russian does not need a separate word for that.