nếu ... thì ...

Placement:

• Nếu sits at the very beginning of the condition clause.
• Thì sits at the beginning of the result clause. The result clause frequently pairs with the future marker sẽ.

Tense Markers:
Generally, do not use the future marker sẽ inside the nếu clause, even if the condition refers to the future. It usually belongs in the thì clause (e.g., Nếu trời mưa, tôi sẽ ở nhà - 'If it rains, I will stay home').

Past and Unreal Conditions:
To express a counterfactual or unreal past condition, nếu remains the standard and most common word, though Vietnamese also uses giá mà (if only) or giá như. The past marker đã is frequently used in the result clause to indicate what would have happened. However, in the nếu clause, đã is often dropped if a time word already establishes the past context (e.g., Nếu hôm qua tôi biết, thì tôi đã ở nhà - 'If I had known yesterday, I would have stayed home').

Spoken Variations:
In casual spoken Vietnamese, nếu is frequently expanded to nếu như or nếu mà.

Dropping words:
In casual spoken Vietnamese, it is very common to drop nếu (relying only on thì). Conversely, dropping thì (relying only on nếu) is perfectly standard in both formal written and spoken Vietnamese. When thì is dropped, it is standard to use a comma in writing or a slight pause in speech to separate the clauses. When thì is used, a comma is often omitted in short sentences, but it is frequently used before thì if the condition clause is long to indicate a natural breathing pause. Additionally, if the subject is the same in both clauses, it is standard to drop the subject in one of the clauses (e.g., Nếu tôi khỏe thì sẽ đi học or Nếu khỏe thì tôi sẽ đi học - 'If I am well, I will go to school'). Furthermore, if the context is obvious (especially for advice or general statements), both subjects are frequently dropped entirely (e.g., Nếu mệt thì nghỉ - 'If you are tired, rest').

Clause Inversion:
When the sentence order is reversed to [Result] + [Condition] (e.g., Tôi sẽ đi học nếu tôi khỏe), the word thì must be dropped entirely.

Negative Conditions:
To form a standard negative condition, simply place không inside the clause (e.g., Nếu trời không mưa... - 'If it does not rain...'). You can also use chưa (not yet) for conditions that haven't happened yet (e.g., Nếu chưa làm xong bài tập thì đừng đi chơi - 'If you haven't finished your homework yet, don't go out'). A very common variation is nếu không thì (or just nếu không) used on its own, which means 'otherwise' or 'if not'.

Important Note:
Do not use thì to mean 'and then' in a standard step-by-step sequence of events. For chronological sequences, use rồi or sau đó. However, thì has other functions, such as marking a topic (thì) or introducing a sudden interruption (e.g., đang ... thì ...).