"Their dog eats."
Translation:Ο σκύλος τους τρώει.
No, they are interchangeable. It's just a preference of the speaker. The diminutive of the mascculine version (ο σκυλάκος) is used to show affection to the dog whereas the diminutive of the neurum version (το σκυλάκι) can be either used for affection or to talk about pups. That is their only difference, i think. Other animal names have both masculine, feminine and neutrum p.e. ο γάτος, η γάτα, το γατί. Then, its a matter of gender but the rule above still applies.
The truth is that you cannot sum up all this because the masculine, feminine or neutrum version of an animal may be used differently. For example you can say for all dogs ο σκύλος or το σκυλί, but η σκύλα is used only for female dogs (or as an insult-bitch), but for cats you can say η γάτα or το γατί for all cats and ο γάτος only or the male cats. So, it's a matter of practice... :/
Without specific information, we have no way of knowing what you mean. Perhaps the "previous lesson" required a different form.
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