[one-man-bucket]

Sorry for being off topic, but I thought the correct word would be "applies" in this sentence. Could anyone educate me, please?

Sinc., a swede

[pmjordan]

The subject in this sentence, "two sources of stress," is third person plural. Verbs are only conjugated with -s for third person singular in English.

One source of stress applies

Two sources of stress apply

[cpeterso]

The subject is "sources", which is plural. You can break it down like this:

  1. Two sources  of stress  that apply to you.
  2. Two sources (of stress) that apply to you.
  3. Two sources             that apply to you.
  4.     Sources             that apply to you.

[udp]

If there was only one source of stress, it would indeed be "A source of stress that (probably) applies to you". But because it's two sources, apply is correct.