If a videogame company names "iSkateRider" a game, can the trademark owners of "iSkate" sue it?

You shouldn't post real names on this public site, you should only disclose these things to your own IP lawyer in confidence. When two trademarks use the same first word, that's more significant, and more likely to be confusing to consumers, then identical 2nd or 3rd words. That consumer confusion is what results in a finding of Infringement, either by a civil court judge or by the Trademark Trial and Appeal Board, which adjudicates procedures to cancel trademarks. But similarity of marks is just part of the inquiry. Are both users' trademarks for the same class of goods, i.e., videogames? Which user has been using their mark longer? Is there any evidence if consumer confusion? If a user has superior rights because of earlier use of their mark than the other user, has a federally registered mark, and the two marks are being used for the same or similar goods, they should consult their own IP lawyer about taking action against the other user. Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

Whether infringement has occurred hinges upon the likelihood of confusion by consumers of the trademarks at issue. To establish a trademark infringement claim or an unfair competition claim a business must show that another entity is using a mark confusingly similar to a valid, protectable trademark. Registering a trademark on the Principal Register in the Patent and Trademark Office constitutes evidence of the validity of the registered mark and of registrant's exclusive right to use the mark on the goods and services specified in the registration. Another party can rebut the presumption that the registered trademark is valid and that registrant is entitled to exclusive use of mark by showing that it was the party which first used the mark, not the registrant. The fundamental tenet of trademark law is that ownership of an inherently distinctive mark is governed by priority of use. You ask "what would a judge say about it?" More facts woud be helpful but in general courts look to eight key factors for guidance in determining the likelihood of confusion In determining the likelihood of confusion in trademark infringement actions the courts look to these eight factors: the similarity of the conflicting designations; the relatedness or proximity of the two companies' products or services; strength of the plaintiff's mark; marketing channels used; the degree of care likely to be exercised by purchasers in selecting goods; the defendant's intent in selecting its mark; evidence of actual confusion; and the likelihood of expansion in product lines. Some factors are much more important than others, and the relative importance of each individual factor will be case-specific. Some of the factors relevant to determining the likelihood of confusion will always be important, such as the similarity of the marks and whether the two companies are direct competitors.

Are the two companies competitors to each other? Keep in mind that "Delta" for faucets can coexist with "Delta" for airlines. A judge will measure what's called "likelihood of confusion." There are elements to that, and the facts of the case will dictate the outcome of the litigation. That said, it's usually a good idea for your attorney to sit down with their attorney and hammer out an agreement before lawsuits get filed.