Learn Turkish: Ben ofisteyim. - I am in the office.

This is because of consonant assimilation. The word ofis ends in s, which is a voiceless or hard consonant (remember the catchphrase Fıstıkçı Şahap). When a suffix starting with d attaches to a hard consonant, the d hardens into a t so it is easier to say. Therefore, the locative suffix -de becomes -te.

What does the -y- do in the middle of ofisteyim?

It acts as a bridge. The word ofiste (in the office) ends with the vowel e. To add the 'I am' suffix -im (which starts with the vowel i), we need a buffer consonant to keep the two vowels from crashing together. The letter y steps in to connect them, giving us ofiste-y-im.

Does -im mean "I am"? Why do we need it if we already have Ben?

Yes, -im is the personal suffix for 'I am'. In Turkish, you attach this 'to be' suffix directly to the end of the word. Because -im already tells us who is speaking, the word Ben (I) is actually optional. You can simply say Ofisteyim and it perfectly means 'I am in the office'.

Why are the suffixes -te and -im used instead of -ta and -um?