A sentence can start with a word other than subject, say a conjunction, conjunctive adverb, or vocative. But there is another category of words such as yes, no, yeah, yep, hmm, huh, well, oh, and ah that can start a sentence. Most of these are used mainly in spoken English and are often derided as filler words.
These words can be sub-categorized into reaction signal and initiator. Whereas initiators mainly start sentences that initiate a conversation, reaction signals mainly start sentences that come as a response. Here are common ones:
Reaction signals: yes, no, yeah, yep, m, um, hm, hmm, uh, huh
Initiators: well, oh, ah, oh well, well then, psst
Examples of reaction signal and initiator in a short conversation:
A: Well, I’m thinking about going to the new cafe downtown. What do you say?
B: Ah, yeah, I heard it’s good. Let’s check it out.
A: Yep, sounds like a plan. Shall we meet there at three?
B: Of course, see you then!
