Wenn ich ein Wort nicht kenne, schaue ich es schnell in einem Onlinelexikon nach.

Because wenn introduces a subordinate clause in German.

In subordinate clauses (introduced by words like wenn, weil, dass, etc.), the conjugated verb usually goes to the end of the clause.

Structure here:

• Wenn (subordinating conjunction)
• ich (subject)
• ein Wort (object)
• nicht (negation)
• kenne (conjugated verb at the end)

So:

• Wenn ich ein Wort nicht kenne, …
  literally: If I a word not know, …

In a main clause, the conjugated verb must be in second position. But “second position” means second element, not second word.

In this sentence, the whole subordinate clause „Wenn ich ein Wort nicht kenne“ counts as one element and stands in the first position. Therefore, the finite verb of the main clause, schaue, must come next:

1. Wenn ich ein Wort nicht kenne, → 1st position (entire clause)
2. schaue → 2nd position (verb)
3. ich es schnell in einem Onlinelexikon nach.

If you did not put the wenn-clause at the beginning, the neutral order would be:

• Ich schaue es schnell in einem Onlinelexikon nach, wenn ich ein Wort nicht kenne.

Ein Wort is in the accusative singular.

Reason:

• The verb kennen takes a direct object in the accusative.
• Neuter noun das Wort has the same form in nominative and accusative:
  
  • Nominative: das Wort / ein Wort
  • Accusative: das Wort / ein Wort

So you see no change in form for neuter nouns here. A genitive singular would be eines Wortes, but there is no genitive in this sentence.

Es is a pronoun that refers back to ein Wort from the first clause:

• Wenn ich ein Wort nicht kenne,
  → that word = es
• schaue ich es … nach.

In English we often drop this:

• If I don’t know a word, I quickly look it up online.

In German you normally keep the pronoun; leaving it out would sound unnatural:

• ✓ Wenn ich ein Wort nicht kenne, schaue ich es schnell in einem Onlinelexikon nach.
• ✗ Wenn ich ein Wort nicht kenne, schaue ich schnell in einem Onlinelexikon nach. (sounds incomplete)

Word order of es:

• Pronouns usually come early in the “middle field”:
  • schaue ich es schnell in einem Onlinelexikon nach
  • You could also say: schaue ich es in einem Onlinelexikon schnell nach, but the given order is very natural.

Nicht usually comes before the part of the sentence that is being negated, but in subordinate clauses with a single verb, it often appears right before the verb, after objects.

Typical pattern in a subordinate clause:

• Subject – object(s) – nicht – verb

Here:

• ich (subject)
• ein Wort (object)
• nicht (negation)
• kenne (verb)

You generally don’t say:

• ✗ Wenn ich nicht ein Wort kenne, …
  This would sound like you’re contrasting “one word” with something else (not one word, but another thing).

Nachschauen is a separable verb (trennbares Verb).

• Infinitive: nachschauen
• The prefix nach is separated and moved to the end of the clause in most main-clause forms:

Examples:

• Ich schaue es nach.
• Schaust du es nach?
• Wenn ich Zeit habe, schaue ich es nach.

In forms where the verb is not in the “second-position-main-clause” slot (e.g. infinitive, perfect, subordinate with zu), it stays together:

• Ich will es nachschauen. (infinitive)
• Ich habe es nachgeschaut. (past participle)
• …, dass ich es nachschaue. (subordinate clause with dass; verb bundle at the end)

In your sentence:

• schaue (conjugated part in 2nd position)
• … nach (separable prefix at the end)

The preposition in can take either:

• Dative → when it’s about location (where?)
• Accusative → when it’s about movement into something (where to?)

Here, it means “in/inside an online dictionary” as a place where you look up the word, so it’s a static location → dative:

• in einem Onlinelexikon (neuter, dative singular)
  
  • Nominative: ein Onlinelexikon
  • Dative: einem Onlinelexikon

Compare:

• Ich suche es in einem Onlinelexikon. (where? → dative)
• Ich gehe in ein Onlinelexikon einloggen. (hypothetical; movement into → accusative)

Schnell is a manner adverb (“quickly”). In German, adverbs normally stand in the “middle field” between the conjugated verb and the stuff that follows or between different complements.

Your sentence:

• schaue ich es schnell in einem Onlinelexikon nach

Order here:

1. schaue (verb)
2. ich (subject)
3. es (pronoun object)
4. schnell (adverb of manner)
5. in einem Onlinelexikon (prepositional phrase)
6. nach (separable prefix)

Other acceptable variants:

• schaue ich schnell in einem Onlinelexikon nach. (adverb before pronoun; slightly different emphasis)
• schaue ich in einem Onlinelexikon schnell nach. (more emphasis on the location first)

All three are grammatically fine; the given version is very natural and neutral. Extreme movements like putting schnell at the very end would sound odd:

• ? schaue ich es in einem Onlinelexikon nach schnell. (unusual)

German distinguishes these verbs clearly:

• kennen – to be familiar with / to know (a person, a thing, a word, a place)

  • Ich kenne dieses Wort nicht. = I am not familiar with this word.

• wissen – to know (a fact, information)

  • Ich weiß nicht, was dieses Wort bedeutet. = I don’t know what this word means.

• verstehen – to understand

  • Ich verstehe dieses Wort nicht. = I don’t understand this word.

In your sentence, the idea is “I don’t know this word / I am not familiar with it,” so kennen is the correct verb:

• Wenn ich ein Wort nicht kenne, …
  = If I don’t know a word (i.e., it’s unfamiliar to me), …

All three can be used when looking something up, but they have nuances:

• nachschauen

  • Very general: to check / to look up / to have a look.
  • Often used for checking information in some source (book, phone, website).
  • Your example: ein Wort in einem Onlinelexikon nachschauen is natural.

• nachschlagen

  • More traditional: to look up in a reference book, especially a dictionary, encyclopedia, etc.
  • Strong association with books or “dictionary-like” sources.
  • ein Wort im Wörterbuch nachschlagen

• nachsehen

  • Also “to check / to have a look”, sometimes more like “to go and see” or “to verify”.
  • Could be used in similar contexts but is a bit broader and often used for checking physical things or details.
  • Ich sehe kurz nach, ob die Tür zu ist.

In the context of an online dictionary, both:

• ein Wort in einem Onlinelexikon nachschauen
• ein Wort in einem Onlinelexikon nachschlagen

are understandable; nachschauen sounds slightly more casual / modern.

German loves compound nouns. When two nouns (or noun + another element) form a single concept, they are usually written together:

• Onlinelexikon = Online + Lexikon → “online dictionary”
• Other examples:
  • Handyvertrag (phone contract)
  • Autotür (car door)
  • Wörterbuch (Wort + Buch)

Sometimes you also see a hyphen:

• Online-Lexikon

This is also correct and sometimes preferred for readability, especially with longer compounds. But „Online Lexikon“ as two separate words is normally incorrect in standard German.

German uses the present tense very often for:

• current actions
• future actions
• and habits / general truths

Your sentence expresses a habit:

• Wenn ich ein Wort nicht kenne, schaue ich es schnell in einem Onlinelexikon nach.
  = Whenever / If I don’t know a word, I (typically) look it up quickly…

You could use future tense, but it would sound less natural here:

• Wenn ich ein Wort nicht kenne, werde ich es in einem Onlinelexikon nachschauen.
  – sounds more like a specific plan or promise, not a general habit.

So the simple present is exactly right for describing a regular behavior.