The US Geological Survey wants to map every human-made structure in the United States, and it is asking volunteers to...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

Sentence 1:

• The US Geological Survey wants to map every human-made structure in the United States,
  • and
• it is asking volunteers to help.

Sentence 2:

• Cassie Tammy Wang and Ashish D'Souza are just two of the many volunteer map editors
  • who [?] to the project since it began in 2012.

Where [?] = contribute / will contribute / have contributed / will be contributing

Understanding the Meaning

Let's start with the first sentence:

• 'The US Geological Survey wants to map every human-made structure in the United States'
  • So there's an ambitious mapping project happening
• 'and it is asking volunteers to help'
  • They're recruiting volunteers to help with this mapping work

Now the second sentence gives us specific examples:

• 'Cassie Tammy Wang and Ashish D'Souza are just two of the many volunteer map editors'
  • These are two specific volunteers being mentioned as examples
  • But they're just two among many volunteers

This is where we have the blank:

• 'who ______ to the project'

Let's look at the choices - they're all forms of "contribute" but in different tenses:

• contribute (present)
• will contribute (future)
• have contributed (present perfect)
• will be contributing (future continuous)

To see what works here, let's read the rest of the sentence and understand what it's telling us!

• 'since it began in 2012'

This is crucial information! Let's understand what this tells us:

• 'Since it began in 2012'
  • The project started at a specific point in the past (2012)
  • The word 'since' signals that we're connecting that past starting point to the present
  • These volunteers have been helping from that past point up until now

The complete picture is:

• The project began in 2012
• These volunteers started contributing at some point after that
• Their contribution has been ongoing from the past into the present

What do we notice about the structure here?

• The phrase 'since it began in 2012' is a time marker that creates a specific requirement
  • When you have 'since' followed by a specific past time, you need a verb form that connects past to present
  • This is what present perfect does - it shows action that started in the past and continues to or has relevance in the present

Also notice:

• 'who' is the subject doing the contributing
  • 'who' refers back to the volunteer map editors (Cassie, Ashish, and others)
  • This is plural, so we need the plural form 'have'

So we need: have contributed

This gives us: 'volunteer map editors who have contributed to the project since it began in 2012' - meaning they started contributing sometime after 2012 and continue to contribute or their contribution is still relevant now.

Grammar Concept Applied

Using Present Perfect with "Since" + Specific Past Time

When you see the word "since" followed by a specific point in the past, you need to use present perfect tense (have/has + past participle). Present perfect tense is specifically designed to connect a past starting point to the present moment.

The pattern:

• Subject + have/has + past participle + since + [specific past time]

Example 1:

• ✓ "She has worked at the company since 2018"
  • Started working in 2018 (past)
  • Still working there now (present)
  • "Has worked" (present perfect) connects these two time points

Example 2:

• ✓ "They have lived in Boston since they graduated"
  • Started living in Boston at graduation (past)
  • Still living there now (present)
  • "Have lived" (present perfect) shows the continuing state

Example 3:

• ✗ "They work here since 2020" - INCORRECT
• ✓ "They have worked here since 2020" - CORRECT
  • Simple present "work" cannot be used with "since" + specific past time
  • Must use present perfect "have worked"

How this applies to our question:

• "volunteer map editors who have contributed to the project since it began in 2012"
  • "Since it began in 2012" = specific past starting point
  • "Have contributed" = present perfect connecting 2012 to now
  • This tells us: they started contributing after 2012 and their contribution continues to or is relevant in the present

Key signal words that require present perfect:

• since + [specific time]: "since Monday," "since 2015," "since the meeting started"
• for + [duration]: "for three years," "for six months" (when describing something still ongoing)