There are various ways the numbers can be transformed from singular to plural. In the case of the ones provided in the question, the answers are: one syllabus, many syllabuses or syllabi; one ox, many oxen; one fish, many fish or fishes; one antenna, many antennae or antennas; one hippopotamus, many hippopotamuses or hippopotami; one octopus, many octopuses; one nucleus, many nuclei; and one focus, many focuses or foci.
We notice in the nouns above that in the cases of syllabus and octopus the word end in -es. In the other cases it ends with -en for ox, and -i for nucleus. In the cases of syllabus, fish, antenna, and hippopotamus, these words in plural end with -s or -i and the speaker is free to pick his choice.
Nonetheless, English is easy because of the limited consonants and vowels. Any learner finds it easy to pick the language and begin to use it as soon as he begins to learn it. However, the problem is when the learner gets to grammar, the rules could be complex and the learner can get confused at the initial stages.