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Если вы обнаружили грибковую инфекцию ногтей (грибки растут на ногтевой пластине или под ней), которая вызывает утолщение и обесцвечивание ногтя, Вам следует обратиться к врачу, поскольку крем не подходит для лечения этих видов инфекций.

If you discover a fungal nail infection (in which fungi grow on the nail plate or under it) which causes thickening and discoloration of the nails, you should consult a doctor, because the cream is not suitable for treating these types of infections.

I translated this sentence from the Russian text of a patient leaflet, and I'm unsure whether I need the comma before "which causes thickening and discoloration of the nails".

On the one hand, it can be considered nonessential information and thus a non-restrictive clause. If you discover an infection of your nails, contact a doctor. And the hallmarks of such infection are the thickenging and discoloration of the nails. Thus, a comma.

On the other hand, how else can a patient discover a nail infection except by noticing the thickening and discoloration of the nails? Thus it must be essential information, and be a restrictive clause. Thus, no comma.

  • At least in US English (it's not true of UK English), if it's an essential clause, you typically use that without a comma; if it's a nonessential clause, you use which with a comma. – Jason Bassford Apr 22 at 17:16
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    Does this answer your question? Should I add a comma before which? – Jason Bassford Apr 22 at 17:22
  • Only you can determine if you want to convey restrictive or nonrestrictive information. If you don't know, and you're translating, your only recourse is to ask the author (if it's critical). Otherwise, do what you think best represents the meaning. – Jason Bassford Apr 22 at 17:24
  • So I thought too. It's subjective. – CowperKettle Apr 22 at 17:25
  • I suspect it's essential. If so, you can also rephrase it: If you discover a fungal nail infection causing [a] thickening and discoloration of the nails … – Jason Bassford Apr 22 at 17:32
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which causes thickening and discoloration of the nails

is non-essential information and should be set off by commas. This is based on an assumption I am making (= Scenario A), but you know the answer to it.

Scenario A: I am assuming that a fungal nail infection does one only thing — it causes thickening and discoloration of the nails.

If this is true, then that clause is non-essential (or non-restrictive). It adds important information, but it is not essential to understanding the intended meaning of the sentence.

The Chicago Manual of Style guide (17th ed.) says

A clause is said to be nonrestrictive (or nondefining or parenthetical) if it could be omitted without obscuring the identity of the noun to which it refers or otherwise changing the intended meaning of the rest of the sentence. Nonrestrictive relative clauses are usually introduced by which (or who/whom/whose) and are set off from the rest of the sentence by commas.

– 6.27: Commas with relative clauses—“that” versus “which”

In Scenario A, everyone knows there is just one thing that happens when you get a fungal nail infection. You don't need that clause to identify which kind of fungal nail infection you are talking about — there is just one.

Scenario B: If there are two or more things that can happen when you get a fungal nail infection, then that clause would be essential information.

A fungal nail infection can do two things. (1) It can thicken and discolor your nails, or (2) it can cause an itchy feeling under your nails.

If this is true, then that clause would be essential information and would not be set off by commas.

If you discover a fungal nail infection that causes thickening and discoloration of the nails, you should consult a doctor. If you discover a fungal nail infection that causes an itchy feeling, soak your fingers in warm water for 2 hours everyday until the itchy feeling goes away.

In Scenario B, that clause is essential because you need that clause to identify the noun (or the noun phrase) — a fungal nail infection. Which kind do you have?


Here is a very basic example:

My brother, John, is an excellent painter.

You have one brother, and you are just providing his name as important information. But "John" is non-essential here because you don't need it to identify which brother you are talking about — you got just one.

My brother John is an excellent painter. (My other brother, Mark, is a bounty hunter.)

In this case, "John" is essential information because you need it to identify which brother you are talking about.


Edit: Here is something regarding "that" and "which".

According to CMoS (17th ed.),

Although which can be substituted for that in a restrictive clause (a common practice in British English), many writers preserve the distinction between restrictive that (with no commas) and nonrestrictive which (with commas).

– 6.27: Commas with relative clauses—“that” versus “which”

In 5.250: Good usage versus common usage, CMoS states

In polished American prose, that is used restrictively to narrow a category or identify a particular item being talked about {any building that is taller must be outside the state}; which is used nonrestrictively—not to narrow a class or identify a particular item but to add something about an item already identified {alongside the officer trotted a toy poodle, which is hardly a typical police dog}. Which is best used restrictively only when it is preceded by a preposition {the situation in which we find ourselves}. Nonrestrictively, it is almost always preceded by a comma, a parenthesis, or a dash. (In British English, writers and editors seldom observe the distinction between the two words.) Is it a useful distinction? Yes. The language inarguably benefits from having a terminological as well as a punctuational means of telling a restrictive from a nonrestrictive relative pronoun, punctuation often being ill-heeded. Is it acceptable to use that in reference to people? Is friends that arrive early an acceptable alternative to friends who arrive early? The answer is yes. Person that has long been considered good idiomatic English. Even so, person who is nearly three times as common as person that in edited English.

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