Tôi nghi lịch đã thay đổi.

No, they are different words.

• nghi = to suspect
• nghĩ = to think

So Tôi nghi lịch đã thay đổi means I suspect the schedule has changed, not just I think...

This is an important spelling-and-tone difference:

• nghi has no tone mark
• nghĩ has the ngã tone mark

Many learners mix these up because they look very similar, but the meanings are different.

Also, in everyday speech, tôi nghĩ... is much more common than tôi nghi... unless you specifically want the stronger sense of suspect.

Yes, it is understandable and grammatically fine, but it can sound a bit concise or slightly formal depending on context.

More natural everyday alternatives might be:

• Tôi nghĩ lịch đã thay đổi. = I think the schedule has changed.
• Tôi nghi là lịch đã thay đổi. = I suspect that the schedule has changed.
• Tôi nghi lịch trình đã thay đổi. = I suspect the schedule/itinerary has changed.

Adding là often makes the sentence flow more naturally when introducing a full clause after nghi.

Vietnamese often omits that when it is clear from context.

So:

• Tôi nghi lịch đã thay đổi.
• Tôi nghi là lịch đã thay đổi.

Both can mean I suspect (that) the schedule has changed.

The version with là is a little more explicit and can sound smoother, especially for learners. The version without là is more compact.

đã usually marks that something happened before now or is already completed.

In lịch đã thay đổi:

• đã tells you the change has already happened
• thay đổi = change / change state

So the meaning is roughly:

• the schedule has already changed
• or more naturally in English, the schedule has changed

Without đã, lịch thay đổi could sound more general or less clearly completed, depending on context.

lịch can mean several related things, including:

• calendar
• schedule
• timetable

In this sentence, schedule is the most likely meaning.

If you want to be clearer, Vietnamese often uses more specific words:

• lịch trình = itinerary, schedule
• thời khóa biểu = timetable/class schedule
• lịch hẹn = appointment schedule

So if there is any chance of ambiguity, a speaker might choose a more specific noun than just lịch.

Because classifiers are not always needed.

Vietnamese often uses a noun by itself, especially when:

• speaking generally
• talking about something already understood from context
• using the noun as the subject of a sentence

So lịch đã thay đổi is perfectly normal.

You might hear cái lịch, but that more often refers to a physical calendar object, like something hanging on a wall or sitting on a desk.

Here, because lịch means schedule, no classifier is needed.

The structure is:

• Tôi = I
• nghi = suspect
• lịch = the schedule
• đã = already / past marker
• thay đổi = change / has changed

So the sentence is basically:

I + suspect + schedule + already + change

A more English-like breakdown would be:

I suspect [the schedule has changed].

Vietnamese often puts the main verb first, then the clause being suspected/thought/etc. after it.

Here thay đổi functions as the predicate meaning to change or to be changed.

In lịch đã thay đổi, it means:

• the schedule has changed
• literally something like the schedule already changed

Vietnamese does not always separate active/passive/state meanings the same way English does. So thay đổi can often cover ideas like:

• change
• be changed
• become different

The exact English translation depends on context.

It could, but it changes the nuance.

• lịch đã thay đổi = the schedule has changed
• lịch đã bị thay đổi = the schedule has been changed

Adding bị often suggests:

• an external agent caused the change
• sometimes an unwanted or negative effect

So if you just want to say the schedule is different now, đã thay đổi is the more neutral choice.

It is possible, but tôi nghĩ is much more common in everyday speech.

Compare:

• Tôi nghĩ lịch đã thay đổi. = I think the schedule has changed.
• Tôi nghi lịch đã thay đổi. = I suspect the schedule has changed.

nghi sounds stronger and more specific: you are not just thinking something; you suspect it based on clues.

In some contexts, nghi can also sound a bit formal, factual, or even investigative.

Some very natural conversational alternatives are:

• Tôi nghĩ lịch đổi rồi.
• Tôi nghĩ lịch đã đổi.
• Tôi nghi là lịch đổi rồi.
• Hình như lịch đổi rồi. = It seems like the schedule changed.

Notes:

• đổi is a shorter everyday verb often used instead of thay đổi
• rồi often gives the sense of already / now it has happened
• hình như is very common for it seems / apparently

So while your sentence is correct, real conversation may use shorter and more idiomatic versions.

Yes—especially nghi versus nghĩ.

A few points:

• Tôi sounds roughly like toy
• nghi begins with ngh-, which in northern and southern speech is pronounced like ng in sing, but at the start of a word
• lịch has a short vowel and a final -ch
• đã has a marked tone
• thay đổi has two syllables, each with its own tone

The biggest learner trap is this pair:

• nghi = suspect
• nghĩ = think

If you mispronounce the tone, people may hear the wrong word.