ידע תדע כי גר יהיה זרעך בארץ לא להם ועבדום וענו אתם ... וגם את הגוי אשר יעבדו דן אנכי ... (טו-יג,יד)
Hakadosh Boruch Hu tells Avraham Avinu about the future trials and tribulations that his children will endure at the hands of the nations of the world. The posuk enumerates two specific evils: forced labor and suffering. Yet, when Hashem promises retribution to these nations, the Torah just says "וגם את הגוי אשר יעבדו" omitting the "וענו" (suffering) part. Why?
One possible answer is that for the extra measure of agony inflicted upon the Jewish people, it is self understood that they will receive punishment. The chiddush here is that even for the back-breaking labor, there will be payback and retribution. Accordingly, we can now understand the meaning of the seemingly extra word "וגם" (see Ramban). Because even the nation that merely subjected Bnei Yisroel to forced labor will be “remembered.”
The Sefer Kehillas Yitzchok quotes another explanation. We find that the word "וענו" can also refer to spiritual pain. Thus, Hashem was saying that a dual tzara will befall us: both physical labor, as well as spiritual degradation. The goyim will cause us to have a decline in our Shemiras HaTorah U’mitzvos. For that, it is obvious that they will pay a heavy price. The chiddush is that they will pay even for the "יעבדו", even for the physical oppression and not just for the spiritual pain.
Rabbeinu Sadia Gaon zt”l points out that we see something fascinating here. The entire Yetzias Mitzrayim, the myriad miracles, the downfall of Egypt - all of these came about due to the two words that Hashem promised - "דן אנכי". Judging by the severity of their payback based on TWO words from Hashem, we cannot begin to imagine the good that is in store for us when Moshiach arrives, based on the multiple prophetic words of our Neviim depicting the heavenly bliss of the ימות המשיח, which is ours to enjoy while we experience Hashem’s nekama on all those who oppressed us throughout the millennia, as the posuk in Tehillim says, "יודע בגוים לעינינו נקמת דם עבדיך השפוך". May we soon be zoche לראות בטוב ה' בארץ חיים