A munkahelyen hosszú megbeszélés volt, de sokat tanultam.

Hungarian normally requires a definite article (a/​az) even when you attach a locative suffix.

• munkahely = workplace, office
• -en = “at” (location suffix)
  So a munkahelyen literally means “at the workplace,” which we naturally translate as “at work” or “in the office.”

megbeszélés is a noun built from the verb megbeszél (“to discuss”):

1. meg- (prefix indicating a completed action)
2. beszél (verb stem “speak/talk”)
3. -és (noun-forming suffix)
   Together, they create megbeszélés, the act of discussing—that is, a meeting or discussion.

Hungarian modifiers (adjectives, numerals) typically precede the noun they describe.

• hosszú megbeszélés = “long meeting”
  Word order in Hungarian is fairly flexible for emphasis, but the most neutral way is always adjective + noun.

volt is the past tense of “to be” (van = he/she/it is). Here it expresses existence in the past:

• volt = “there was”
  So hosszú megbeszélés volt means “there was a long meeting.” If it were happening now, you’d say hosszú megbeszélés van (“there is a long meeting”).

Yes. Hungarian allows you to swap the verb and noun phrase in “there is/was” sentences:

• A munkahelyen hosszú megbeszélés volt. (neutral)
• A munkahelyen volt hosszú megbeszélés. (still correct, slightly more emphatic on the fact that it occurred)
  Both are understood; the difference is mostly about which part you emphasize.

de means “but” or “however.” It introduces contrast between the two clauses.
Placement: directly before the second clause or attached to the first verb of that clause.

• …, de sokat tanultam. (neutral)
  You wouldn’t normally move de to the very end.

sokat is the accusative form of sok (“much/a lot”). When you say sokat tanultam, “a lot” is the object of tanultam (“I learned”), so it takes the -t for accusative.

• sok (dictionary form/adjective)
• sokat = “a lot” (as a pronoun/object)

Hungarian is a pro-drop language: the verb ending -tam already tells you the subject is “I,” so you don’t need én.
To add emphasis: place én before the verb or stress it in speech.

• Én tanultam sokat. = “I (and not someone else) learned a lot.”