A lot of foreign people might think it's strange, but Japanese often say "わかりました（I understood.）", even though we are talking about present.
"わかりました" is past tense, so it is natural you think it should be present tense,"わかります（I understand）" when you are being instructed by someone now. However, in Japanese discourse like when you express your understanding to someone's instruction or indication, it is common that you response with past tense. Then, when should you say "わかります(I understand.)"? There are about 2 ways of the situation you have better use present tense.
A:Do you tell the meaning of this Russian word?
B:Yes, I understand.
Like this case, when you can understand the meaning of stuff or contents, especially in the midst of the indication, you better say "わかります".
A:My husband never tries to listen to me! Disgusting!
B:I understand. Same here.
Like above, when you want to show that you can understand someone's feelings or situations, or when you want to sympathize with someone, you should use "わかります".
In short, when you express your full comprehension about something, especially after someone has finished his explaining, you have to use past tense. On the other hand, when you want to show your understanding each time or temporarily, particularly amid instruction, you should response with present tense.