Dear Rabbi I understand that when reading some Hebrew words, one is supposed to add extra emphasis on letters (as indicated by the line on the top of the letter in the Artscroll siddur). Could you explain the reasons behind this?

ב"ה Shalom Since I don't have an Artscroll siddur I can only guess what you are referring to. In general ,there are places in the siddur where you have to be careful not to say words together, lest it give the wrong meaning and instead of praising Hashem we would commit blasphemy. There are words that we have to know if it is a regular "kamatz" or is it a small kamatz" which changes pronunciation. There are words, such as "echad" in Shema that we have to articulate and there are words in which the first syllable is emphasized and there are words in which the last syllable is emphasized. I hope some of these suggestions cover your question. If not you're welcome to question again. Shanna Tova