Hi Michael! Nice meet you. Good question, but they're not easy to explain, but I'll try ^^ Now, I'll explain you about "masu", "teimasu","masita" and "teimashita". ①masu It means a present tense. Ex. Watashi wa asagohan wo tabemasu. It means "I have breakfast.". Also it's used as a custom. Watashi wa maiasa hachiji ni asagohan wo tabemasu. "Maiasa" means "every morning" So it means "I have breakfast at eight every morning.". Also it's used as kinds of future tense. Kyou watashi wa kuji ni asagohan wo tabemasu (mainichi watashi wa sore wo hachiji ni tabbemasu ga). It's means "I'm going to have breakfast at nine today (though I have it at eight every day.). ②teimasu It means present progressive tense. Ex. Watashi wa asagohan wo tabeteimasu (now). It means "I'm eating breakfast (now).". ③mashita It means a past tense. Ex. Watashi wa kesa asagohan wo tabemashita. "Kesa" means "this morning". So it means "I had breakfast this morning.". ④teimashita It means a past progressive tense. Ex. Sonotoki watashi wa asagohan wo tabeteimashita. "Sonotoki" means "at that time/then". So it means "I was eating breakfast at that time.". I hope my explanation help you. I've shown some words lists in Japanese and English for foreign learners on "Memo" of my profile. Also I've shown some word games in Japanese and English on "Activity" of my profile. Have fun with leaning Japanese ^^

'Tabemasu' means 'eat' (or 'will eat'), while 'tabeteimasu' means 'am/are/is eating'. わたしは毎日納豆を食べます。→ I eat natto every day. あしたも納豆を食べます。 → I will eat natto tomorrow, too. いま納豆を食べています。 → I am eating natto now. 'Tabemashita' means 'ate' (or 'have eaten') and 'tabeteimashita' means 'was/were eating'. 昨日も納豆を食べました。 → I ate natto yesterday, too. 今日はもう納豆を食べました。→ I have already eaten natto today. そのとき納豆を食べていました。 → I was eating natto then.