An user Galician
says…

É
Auxiliary
 positiva  cando
Subordinating conjunction
  os
Determiner
  síntomas
Noun
 descritos  previamente
Adverb
  aparecen
Verb
  no
 primeiros  trinta
30
Numeral
 segundos  do
 test.

It is positive when the previously described symptoms appear within the first thirty seconds of the test.
Words and sentences
New
cando

New
os

New
síntomas

plural of síntoma

New
previamente

previously (at an earlier time)

New
aparecen

third-person plural present indicative of aparecer

New
no

30
New
trinta

thirty; 30

New
do

of the; from the; 's

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