Spanish language learning forums - Explicármelo y Explícamelo -acentos?

Each verb form has a specific syllable that has the primary stress (or "tonic accent"). Which syllable it is never changes, regardless of how many object pronouns one adds to the end.

When the syllable that has the tonic accent is not the "correct" syllable (last syllable when the word ends in a consonant that is not 'n' or 's', second-to-last when the word ends in a vowel letter, 'n' or 's'), one MUST write an accent mark on the syllable that has the tonic accent.

Looking at your examples:

The infinitive explicar has the tonic accent on the last syllable, which matches the rule; no written accent required.

Add a single object pronoun: explicarlo also matches the rule, no written accent required.

Add two object pronouns: explicármelo does not match the rule; a written accent is required.

The imperative form for tú is explica, which has the tonic accent on the second-to-last syllable, which matches the rule, so no written accent is required.

Add a single object pronoun: explícalo does not match the rule, so a written accent is required.

Add two object pronouns: explícamelo also does not match the rule, so a written accent is required.

You said that one of your roommates said that it sounds very weird that the accent goes over the "i" in the imperative form. Is that roommate from Mexico, or from another country? I ask because in some countries some people prefer vos instead of tú as the second person singular informal pronoun, and in some of those areas the imperative forms for vos are different from the forms for tú. For example, in both Argentina and Nicaragua the vos imperative is always identical to the infinitive with the final "r" missing, so explicá (does not match the rule, so written accent required), explicalo (matches the rule, no written accent required), and explicámelo (does not match the rule, so written accent required).