How do I recognise the difference between a gerund and a gerundive?

There are a few ways that I have taught myself to remember the difference between these, as they can look very similar.
A gerundive is a passive adjective (think of the -ive endings). You can also look for the letters -nd- in the stem of the verb e.g. portandus. Gerundives will decline in the same way as adjectives, so it will change in form to agree with the gender, number and case of the noun it is being associated with. It declines in a 2-1-2 pattern (feminine, masculine, neuter), the same way as bonus, -a, -um. Gerundives are usually translated into English with the words 'to be' and then the past participle. E.g. habendus, -a, -um = to be had.
Gerunds on the other hand are verbal nouns and they always have similar endings. Nominative and plural gerunds do not exist. You can recognise an accusative gerund by the -ndum ending; a genitive gerund by the -ndi ending; and a dative or ablative gerund by the -ndo ending. Gerunds are translated into English mostly with an -ing ending. E.g. vivendi = of living