Is "在" necessary in this case?
In English we use be+ verb (ing form) to indicate present, continuous action. In Chinese my understanding is that 在 is used somewhat similarly, like in the sentence below:

医生在诊患者

My question is: Is it also grammatically correct to simply say: 医生诊患者 (ie., leave out "在")?

Or, to put it another way, is "在" REQUIRED in such a sentence? If we leave it out does it change the sense, or make the sentence simply wrong, grammatically speaking?

Question

Is "在" necessary in this case?
In English we use be+ verb (ing form) to indicate present, continuous action. In Chinese my understanding is that 在 is used somewhat similarly, like in the sentence below:

医生在诊患者

My question is: Is it also grammatically correct to simply say:   医生诊患者  (ie., leave out "在")?

Or, to put it another way, is "在" REQUIRED in such a sentence? If we leave it out does it change the sense, or make the sentence simply wrong, grammatically speaking?

Answer

在 : 置於動詞之前，表示動作正在進行。
如：「我在聽音樂。」﹑「妹妹在唱歌。」﹑「醫生在診患者。」。
這近似英語的 進行式。

去掉 「在」 之後， 語意變成 一般 簡單的敘述。
如：「我聽音樂。」﹑「妹妹唱歌。」﹑「醫生診患者。」。
這近似英語的 簡單式。

Answer

Exactly, 在 is the equivalent of BE + verb(ing) in Chinese. Sometimes it would be difficult for us to understand what you exactly mean if you leave it out. In addition, your sentence 医生诊患者 sound VERY awkward. Normally we say the following,

医生在看病（呢）。
医生在看诊（呢）。
医生在看病人（呢）。
......

If you leave 在 out of these sentences, actually we don't know what you trying to say at all.

医生看病 X（ We don't know if you want to tell us that the doctor is examining patients OR you're trying to say that a doctor is someone who examine patients. The sentence without 在 would be incomplete and confusing.)

医生在看病= 医生看病呢 = 医生在看病呢✓ （ The doctor is examining patients.)

As you can see, a single word sometimes play an important role in a sentence.

I hope that the explanation above was clear to you. If not, I'd be happy to explain further.

Answer

醫生在看病 的 在 是 副詞  這個副詞的用法是唯一的
醫生在醫院 的 在 是 介係詞  做介係詞使用的 在 則有以下三種用法

1 表示時間。如：「他喜歡在晚上看書。」 
2  表示處所﹑位置。如：「人生在世」﹑「他不在家」。 
3  表示範疇。如：「在心理學方面的研究，他是個傑出的專家。」

Answer

医生  在  医院。