"I seldom understand you."
Translation:Zřídka tě chápu.
The short pronouns (ie. the word "ti" in your sentence) are often clitics and because of that, they need to be placed in the second position. (Zřídka ti rozumím./ Rozumím ti zřídka.)
If you need to use more clitics in one sentence (for example when writing a sentence in the past tense) - here is the order of their placement.
Zřídka ti rozumím. = I seldom understand you. It's a translation of the original English sentence.
The verb "chápat" takes accusative - Chápu tě. (I understand you.); the verb "rozumět" takes dative - Rozumím ti. (I understand you.)
There is little difference between the two verbs, often they are interchangeable. "Chápat" is used when you understand logically deeper into the problem and you can apply to similar problems, while "rozumět" is used when you understand the surface, eg. the words that you are told. There is not a strict line, so this is just to give you an idea why you can say "Chápu českou gramatiku. I understand Czech grammar." and you can say "Rozumím česky. I understand Czech."
I love Czech, both in terms of language and the culture and people. However my abiding memory of learning it at UK in Prague was learning some rule one week, then the next learning rules when the rules we learnt the previous week don't apply! I understand Czech confuses native Czechs so non natives don't have much chance really!