Mercurius quondam mendāciīs omnibus et fraudibus in currū positīs, ūniversam terram circumībat. Ut vērō ad Ārabum regiōnem pervēnit, currus diffractus est, ita ut onus omne dēcideret. At Ārabēs, ut aliō trānsīret, nōn permīsēre; adeō ea gēns nec quae commūnia sunt hominibus facere potest.
SOURCE: This comes from the Latin translation that accompanies De Furia's edition of the Greek Aesopic corpus, published in 1810 and available at GoogleBooks. This is fable 349 in De Furia; for other versions, see Perry 309.
READ OUT LOUD. Choose which marked text you prefer to practice with - macrons (above) or accent marks (below) - and read the text out loud until you feel comfortable and confident. Then, try reading the unmarked text at the very bottom. It should be easy for you after practicing with the marked texts. :-)
ACCENT MARKS. Here is the text with accent marks, plus some color-coding for the words of three or more syllables (blue: penultimate stress; red: antepenultimate stress):
Mercúrius quondam mendáciis ómnibus et fraúdibus in curru pósitis, univérsam terram circumíbat. Ut vero ad Árabum regiónem pervénit, currus diffráctus est, ita ut onus omne decíderet. At Árabes, ut álio transíret, non permisére; ádeo ea gens nec quae commúnia sunt homínibus fácere potest.
UNMARKED TEXT. Here is the unmarked text - after practicing with the marked text that you prefer, you should not have any trouble with the unmarked text:
Mercurius quondam
mendaciis omnibus et fraudibus
in curru positis,
universam terram circumibat.
Ut vero
ad Arabum regionem pervenit,
currus diffractus est,
ita ut onus omne decideret.
At Arabes,
ut alio transiret, non permisere;
adeo ea gens
nec quae communia sunt hominibus
facere potest.
IMAGE. Here is an illustration for the story (image source), showing the god Hermes:
