Which sounds natural? 1. Lemons are yellow. 2. The lemon is yellow. 3. A lemon is yellow.
You say,"I like lemons.", but if you use the word "lemon" as a subject of a sentence, how do you use it?
Some say that "the lemon" refers to a definite, specific lemon. This need not be the case. It can legitimately refer to the class of lemons generally, especially in formal or scientific writing: "The lemon is a fruit native to Asia and is widely cultivated due to its many uses." ( http://www.antioxidant-fruits.com/lemons-and-insomnia.html ) Cf: "The platypus is among nature's most unlikely animals." ( http://www.nationalgeographic.com/animals/mammals/p/platypus/ )
However, you asked which sentence sounds natural, so I would prefer "lemons are yellow" and "a lemon is yellow" in everyday use.
There are many rules of articles but the basic ones are:
We often use the indefinite article a/an :
With countable nouns when the hearer doesn't know exactly which one we are referring to. They are researching for a paper. I saw a good film last week
To say something or attribute that all the members of that kind have A man needs love// A lemon is yellow
To show that something is one of particular group. *He is a teacher at London School of Economics *
To say what the job of someone or who is that someone.He is a doctor
We use the definite article the
when we want to talk about something is more certain that speaker and hearer know which it is. Have you seen the car key?. Or to talk about something is somehow unique or, let me say, has a particular preliminary image that comes immediately to our minds. The sun is a huge star // The lemon is yellow
Finally, no article is needed with plural and uncountable nouns to talk about things in general. I bought blue shoes yesterday. Birds eat worms. // Lemons are yellow
Consider the difference here:
They are running after a little elephant
I went to the zoo and saw the elephants
Asian elephants live in Nepal
My friend, your examples are all correct. Of course I don't really get you by natural. I just would like to explain the concept where each sentence makes sense:
1. Lemons are yellow.: This is a fact or opinion about lemons, the author expresses, by which they mean, that the crop lemon is yellow generally.
2. The lemon is yellow.: The response to this statement would be like: "which/what lemon!!?"
3. A lemon is yellow.: This is equivalent to the first sentence; a general statement.
What do you mean by natural? All the three statements are correct and are natural.
When you say "lemons" you mean this fruit in general. When you say "the lemon" you mean some particular lemon. When you say "a lemon" you refer to any lemon in general.
General speaking, yes, “Lemons are yellow” is all right. “ the lemon is yellow”, you are talking about a specific lemon which already you’ve talked about it and is known to the listener. “A lemon is yellow” sounds talking about it in general concept, then the first sentence sounds more natural, but not incorrect.