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I am writing an historical novel, and I try to have my characters speaking and writing as everybody did at the time. But I don't know when we in the US began to use practice as a verb instead of practise.

Could you give me an approximate date?

Samuel Johnson, A Dictionary of the English Language (1756) spells the verb practise and the noun practice. Johnson also includes entries for the related noun forms practisant, practiser, and practitioner.

Noah Webster, A Compendious Dictionary of the English Language (1806) spells both the verb and the noun practice, and includes entries for the related nouns practicer, practisant, and practitioner. So what did Webster think he was accomplishing by changing the verb practise and the noun practiser to practice and practicer while leaving the other words alone?

The answer is that Webster was trying to uphold several not entirely compatible principles of language, each having a different force. The most basic principle, you might reasonably suppose, was that of uniformity. In his preface to the Compendious Dictionary, Webster says this:

Nor ought this principle of uniformity to be violated; for uniformity in the classes of words is the most convenient principle in the structure of language, and whatever arbitrary rules the learned may frame, the greatest part of men will be governed by habits of uniformity. To these habits we are indebted for all the regularity which is found in our own language or in any other.

For this reason, rather than from a rigid adherence to the originals, we ought to write defense, pretense, offense, recompense, &c. with s instead of c; for we always use that letter in the derivatives, defensive, offensive, pretension, recompensing.

But uniformity across a class of words is a far less monolithic principle than you might suppose. Though Webster wanted to convert defence to defense, offence to offense, pretence to pretense, and recompence to recompense, he showed no similar interest with regard to difference, eminence, preference, and sequence.

And though he rejected separate spellings for practice (noun) and practise (verb), he didn't try to force practitioner to become practicioner. The reason he accepted the non-uniformity of practitioner and practisant in the midst of an attempt to unify the spelling of practice, I suspect, is that he was actually rather conservative on the question of sweeping away old spellings that could claim to reflect accurately the historical development of a word family.

One might wonder, for example, why Webster was inclined to change the verb to practice rather than choosing to change the noun to practise. Here I think, his regard for etymology played a major role, as he cited the Spanish practica, the Italian pratica, the French pratique, and the Greek πραχτιχη (along with πρασσω and πραττω, admittedly) in the 1828 American Dictionary of the English Language. But in the 1828 dictionary, he also made an argument for practice as the spelling of the verb by analogy to notice:

PRACTICE v.t. From the noun. The orthography of the verb ought to be the same as of the noun; as in notice and to notice.

Interestingly, Johnson's 1756 Dictionary has an entry for notice as a noun but no entry under any spelling for the verb form.

Webster may also have been influenced in favor of the spelling practice by his view of the influence of Norman French on Saxon English after the Conquest. Again from the preface to the 1806 Compendious Dictionary:

The Norman conquest however effected a change in the power of c, and established it as the equivalent of s before e, i, and y.

It would follow (under Webster's analysis) that the Norman influence gave English practice with an s sound but could not support practicant with an s sound, meaning that the spelling of that word had to be practisant.

Conclusion

In any event, the American adoption of the spelling practice for the verb that was spelled practise in England is directly attributable to Noah Webster, who made practice the only spelling that his dictionaries listed throughout the nineteenth century, starting with his first dictionary, the 1806 Compendious Dictionary of the English Language.

Underscoring Webster's responsibility for the U.S. adoption of practice as the spelling of the verb is the fact that his one real rival in the American dictionary trade, Joseph Worcester, consistently listed practise (not practice) as the spelling of the verb form of the word, and practiser (not practicer) as the derivative noun—for example, in A Comprehensive Pronouncing and Explanatory Dictionary of the English Language, revised edition (1845), and in A Comprehensive Pronouncing and Explanatory Dictionary of the English Language, [further] revised edition (1871).

Thanks to the dictionary war between Webster and Worcester, an American might spell the verb practice as early as 1806 with the blessing of Webster, or spell the verb practise at least as late as 1871 with the blessing of Worcester's executors.

Practice appears in both the 1913 and 1828 versions of Webster's unabridged dictionary, so that spelling was in use before 1828.
Practise only appears in the 1913 version.