We’re building a stone memorial for the souls of soldiers who died in battle.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means condolence, so the verb version is to mourn for or to condole.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You haven't learned this reading yet, so here's a mnemonic to help you:
You seem to mourn for things you really care about. Well then you would mourn for your toe mural (とむら). You had a mural made for your favorite toe so if anything were to happen to it, you would want to mourn for it.