Choosing between "which" and "what" when asking questions in English can be tricky, but there is a clear rule that makes it simple. Understanding this difference will help you sound more natural in professional and everyday conversations, especially in work environments where precise communication matters.

When should you use "which" in English questions?

The word "which" is an interrogative adjective used when you are asking someone to make a choice from a limited set of options [0:48]. The speaker already knows or defines the possible answers. Here are some examples from the lesson:

• "Which computer is yours, the MacBook or the HP?" — only two options.
• "Which room is Paul currently working in?" — imagine an office with only three rooms [1:05].
• "There are five programs. Which program are they joining?" — a defined, small group [1:18].

Notice that in every case, the number of choices is specific and countable. The listener picks from a known list.

When should you use "what" in English questions?

The word "what" is also an interrogative adjective, but it is used when the options are unlimited or very broad [1:28]. There is no predefined set to choose from:

• "What university did you attend?" — hundreds of universities exist.
• "What tools do you use at work?" — countless tools available [1:43].
• "What app do you recommend us to download?" — millions of apps to choose from [1:50].

The key difference is straightforward: limited options call for "which"; unlimited options call for "what".

How do "which" and "what" work in a real conversation?

The lesson presents a practical phone call between a customer named Lauren and a sales representative named Axel at an internet service company [2:18]. This dialogue puts both adjectives into action:

• "What plans do you have available?" [2:35] — Lauren does not know how many plans exist, so the options are open.
• "Which additional service would you like to include, telephone or TV services?" [3:02] — Axel offers only two choices, making "which" the correct word.
• "What city are you located in?" [3:33] — There are many cities, so "what" fits perfectly.

Two more examples appear later in the conversation that reinforce the pattern:

• "Which plan do you prefer?" [3:58] — Axel has narrowed it down to two plans: 500 Mbps and 1 Gbps.
• "Which app do you prefer?" [4:22] — The company lists only four streaming apps: Netflix, HBO Max, Disney+, and Amazon Prime.

How can you practice this rule?

A helpful strategy is to ask yourself one question before choosing: "Do I know the exact options?" If yes, use "which." If the possibilities are wide open, use "what."

Try applying this in your daily work routine. For example:

• "What software does your team use?" — broad question.
• "Which meeting room should we book, A or B?" — limited choice.
• "What time works best for you?" — many possible times.
• "Which shift do you prefer, morning or afternoon?" — two defined options.

Why does this matter in professional English?

Using "which" and "what" correctly signals that you understand the context of your question. In customer service calls, team meetings, or emails, picking the right interrogative adjective makes your communication clearer and more professional. It also helps the listener respond faster because they immediately understand whether they are choosing from a list or giving an open answer.

If you found this useful, try writing your own examples using both words and share them in the comments.