importance and difficulty of a distinction. These factors vary from language to language. In French, for example, the tag question n'est ce pas? is added to sentences about as often as the corresponding tag questions are used in English. Yet n'est ce pas? requires much less practice than is needed to master English isn't it?, won't it?, won't they?, can't I?, haven't you?, mustn't she? and so forth. On the other hand, changing from present to past tenses in the best-known European languages including English is troublesome: get, got, but set, set; sink, sank, but think, thought. In Swahili this difference is always made by replacing the prefix na by the prefix li. And in some languages, the verb doesn't change to show tense at all. A French speaker, whose definite and indefinite articles work something like the and a in English, will need less drill on these words than will a speaker of Russian, whose language lacks articles altogether.
A difficult manipulation which is however infrequent and relatively unimportant is the relationship between:
| We waited four hours. | Seldom have we waited so long. |
| I ate fourteen pancakes. | Seldom have I eaten so many. |
Points like this will not be made the subject of 'routine manipulation.' They are best handled by writing manipulative drills ahead of time, as is usually done in the preparation of language textbooks.
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