"It is his dog."
Translation:Είναι ο σκύλος του.
"Aυτό είναι το άλογό μου." Ok, here you have "αυτό" this is neuter and "το αλογό" is also neuter. So, it's correct.
But here you have "Αυτό" again neuter BUT "ο σκύλός" is masculine. So, you need to use "Αυτός" which is masculine also.
The correct sentence would be: "Αυτός είναι ο σκύλος."
To make translating much easier and with fewer mistakes just follow these hints.
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By typing the correct sentence each time you will learn it more easily so don't hesitate to use the hints.