What are two forms of the same word called?

It starts with an S.

Or possibly starts with an H.

Need more explanation here. Do you mean, for example, the “noun form” and the “verb form”? Or two words that are spelled alike but mean different things?

A dictionary would answer this question.

Or two words that are spelled differently and sound alike; viz: new and knew, brake and break, broccoli and cauliflower.

broccoli and cauliflower???

Need more information. It could be Synonym, Homonym, participles, plurals, different tense, different mode, etc., etc., etc.

@Yetanotheruser: When they were young, my kids thought they were two forms of the same thing.

Homophone also.

In looking for an answer to the OP’s Q, I came across a word in this family (of cognates, doublets and allusions—I have no idea what is meant by “prejoration”—and synonyms, homonyms and antonyms) called “paronyms”. A cool word that those of us interested in words should know about. I did not know that before.

Paronym (from Google Definitions)
Definitions of Paronym on the Web:

* A paronym or paronyme in linguistics may refer to two different things: # A word that is related to another word and derives from the same root, e.g. a cognate word; # Words which are almost homonyms, but have slight differences in spelling or pronunciation and have different meanings. ...
en.wikipedia.org/wiki/Paronym

* A word derived from the same root, or with the same sound, as another word
en.wiktionary.org/wiki/paronym

* A word formed from another word in a foreign language. Many fragrances take advantage of both the cachet of French and the fact that English and French share a considerable vocabulary. ...
www.catchwordbranding.com/resources/glossary.php

* word formed from a foreign word
www.thutong.doe.gov.za/ResourceDownload.aspx

Wait! I’ve got it! It’s called a

Or do you mean alternate spellings, such as spectre and specter or honor and honour?

Afterthought: Are you taking a multiple-choice quiz for which you are requesting an answer? If so, the answer is in your coursework.