"The child eats bread and cheese."

[prastibria]

Ηοw do I know whether to use 'το' or 'o' for 'the'?

[_Dimitris_]

The ending of the word indicates (99% of cases) the gender. So we have masculine substantives (ending in -ος, -ας, -ης, -ούς) which take the article "o", feminine substantives (ending in -η, -α, -ου, -ω, -ος) which take the article "η", and neutral substantives (ending in -o, -ι, -μα, -μο) which take the article "το".

[prastibria]

Thanks! That is really helpful. :)

[TobyBartels]

Did you mean to list -ος again among the common feminine endings?

[greekdude_]

There are some nouns and adjectives that keep the -ος ending in their feminine form. For example, many occupation names: ο/η γιατρός (doctor), ο/η δικηγόρος (lawyer), ο/η βουλευτής (member of the parliament)