This seems consistent. "Already" is an example of a time expression, so simple past is required. "I already told Mark that when he arrived, we would go out for dinner.
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AnonymousI already told Mark that when he would arrive, we would go out for dinner.Let us recast the sentence in direct speech, that is, you are speaking to Mark, "When you arrive (simple present tense is most suitable here - I wonder you can use any other tense), we shall/will go out for dinner."
CalifJimLike all future forms (will), Future in the Past (would) cannot be used in (subordinate) clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. In stead of using Fut
Cool Breeze"I wanted to know if the train would arrive on time" is wrong?No. It's not wrong at all. The case simply doesn't come up within the quoted material.
CalifJim Pedagogically, however, if we were to follow the method presented at that web site, we might find it a sound practice to introduce the student to smaller portions of a complicated rule such as this, with time to practice each aspect of it, before moving on to other portions.)I feel that the above is the most logical explanation.