Learn German: Ich treffe heute meinen Partner. - I am meeting my partner today.

In this sentence, your partner is the direct object (the person receiving the action of being met). This means it is in the accusative case. Since Partner is a masculine noun, the possessive word mein must take the masculine accusative ending -en, giving us meinen.

Why does the time word heute come before the object meinen Partner?

German word order usually places time words (like heute) directly after the conjugated verb and before the direct object. While putting heute at the end of the sentence is understood, putting the time before the object is the standard, natural way to build sentences in German.

Does the verb treffen normally change its 'e' to an 'i'? Why doesn't it do that here?

You might be remembering that treffen (to meet) is a strong verb with a vowel change. However, that change only happens in the du (triffst) and er/sie/es (trifft) forms. For ich, it stays regular, so it is just ich treffe.

Since Partner is an English word, do I treat it differently in German?